feat(Frame): introduce Frame.select

This patch adds `Frame.select` method that does the same functionality as
former `Page.select`, but on a per-frame level.

The `Page.select` method becomes a shortcut for the ÷main frame's select.

Fixes puppeteer#1139

docs/api.md

@@ -128,6 +128,7 @@
   * [frame.isDetached()](#frameisdetached)
   * [frame.name()](#framename)
   * [frame.parentFrame()](#frameparentframe)
+  * [frame.select(selector, ...values)](#frameselectselector-values)
   * [frame.title()](#frametitle)
   * [frame.url()](#frameurl)
   * [frame.waitFor(selectorOrFunctionOrTimeout[, options[, ...args]])](#framewaitforselectororfunctionortimeout-options-args)
@@ -932,13 +933,15 @@ Shortcut for [page.mainFrame().executionContext().queryObjects(prototypeHandle)]
 - returns: <[Promise]<[Array]<[string]>>> Returns an array of option values that have been successfully selected.
 
 Triggers a `change` and `input` event once all the provided options have been selected.
-If there's no `<select>` element matching `selector`, the method throws an error. 
+If there's no `<select>` element matching `selector`, the method throws an error.
 
 ```js
 page.select('select#colors', 'blue'); // single selection
 page.select('select#colors', 'red', 'green', 'blue'); // multiple selections
 ```
 
+Shortcut for [page.mainFrame.select()](#frameselectselector-values)
+
 #### page.setContent(html, options)
 - `html` <[string]> HTML markup to assign to the page.
 - `options` <[Object]> Navigation parameters which might have the following properties:
@@ -1504,6 +1507,19 @@ If the name is empty, returns the id attribute instead.
 #### frame.parentFrame()
 - returns: <[Frame]> Returns parent frame, if any. Detached frames and main frames return `null`.
 
+#### frame.select(selector, ...values)
+- `selector` <[string]> A [selector] to query frame for
+- `...values` <...[string]> Values of options to select. If the `<select>` has the `multiple` attribute, all values are considered, otherwise only the first one is taken into account.
+- returns: <[Promise]<[string]>>
+
+Triggers a `change` and `input` event once all the provided options have been selected.
+If there's no `<select>` element matching `selector`, the method throws an error.
+
+```js
+frame.select('select#colors', 'blue'); // single selection
+frame.select('select#colors', 'red', 'green', 'blue'); // multiple selections
+```
+
 #### frame.title()
 - returns: <[Promise]<[string]>> Returns page's title.
 

lib/FrameManager.js

@@ -406,6 +406,26 @@ class Frame {
     }
   }
 
+  /**
+  * @param {string} selector
+  * @param {!Array<string>} values
+  * @return {!Promise<!Array<string>>}
+  */
+  async select(selector, ...values){
+    return await this.$eval(selector, (element, values) => {
+      if (element.nodeName.toLowerCase() !== 'select')
+        throw new Error('Element is not a <select> element.');
+
+      const options = Array.from(element.options);
+      element.value = undefined;
+      for (const option of options)
+        option.selected = values.includes(option.value);
+      element.dispatchEvent(new Event('input', { 'bubbles': true }));
+      element.dispatchEvent(new Event('change', { 'bubbles': true }));
+      return options.filter(option => option.selected).map(option => option.value);
+    }, values);
+  }
+
   /**
    * @param {(string|number|Function)} selectorOrFunctionOrTimeout
    * @param {!Object=} options

lib/Page.js

@@ -784,17 +784,7 @@ class Page extends EventEmitter {
    * @return {!Promise<!Array<string>>}
    */
   async select(selector, ...values) {
-    return await this.$eval(selector, (element, values) => {
-      if (element.nodeName.toLowerCase() !== 'select')
-        throw new Error('Element is not a <select> element.');
-      const options = Array.from(element.options);
-      element.value = undefined;
-      for (const option of options)
-        option.selected = values.includes(option.value);
-      element.dispatchEvent(new Event('input', { 'bubbles': true }));
-      element.dispatchEvent(new Event('change', { 'bubbles': true }));
-      return options.filter(option => option.selected).map(option => option.value);
-    }, values);
+    return this.mainFrame().select(selector, ...values);
   }
 
   /**