Support HEADLESS env variable for unit tests

This patch makes it possible to run unit tests in non-headless
mode with the following command:

```
HEADLESS=false npm run unit
```

CONTRIBUTING.md

@@ -43,14 +43,19 @@ To run puppeteer tests, use:
 npm run unit
 ```
 
-To run phantom-shim against phantomjs tests, use:
+To run all tests, including lints:
 ```
-npm run test-phantom
+npm test
 ```
 
-To run both puppeteer and phantom_shim tests, use:
+To debug unit tests:
 ```
-npm test
+npm run debug-unit
+```
+
+To run unit tests in non-headless mode:
+```
+HEADLESS=false npm run unit
 ```
 
 ## DEBUG module

lib/Page.js

@@ -555,6 +555,7 @@ class Page extends EventEmitter {
       let node = document.querySelector(selector);
       if (!node)
         return null;
+      node.scrollIntoView();
       let rect = node.getBoundingClientRect();
       return {
         x: (rect.left + rect.right) / 2,

test/test.js

@@ -30,6 +30,7 @@ let EMPTY_PAGE = PREFIX + '/empty.html';
 let HTTPS_PORT = 8908;
 let HTTPS_PREFIX = 'https://localhost:' + HTTPS_PORT;
 
+const headless = (process.env.HEADLESS || 'true').trim().toLowerCase() === 'true';
 if (process.env.DEBUG_TEST)
   jasmine.DEFAULT_TIMEOUT_INTERVAL = 1000 * 1000 * 1000;
 else
@@ -50,7 +51,7 @@ describe('Puppeteer', function() {
   let page;
 
   beforeAll(SX(async function() {
-    browser = new Browser({headless: true, args: ['--no-sandbox']});
+    browser = new Browser({headless, args: ['--no-sandbox']});
     const assetsPath = path.join(__dirname, 'assets');
     server = await SimpleServer.create(assetsPath, PORT);
     httpsServer = await SimpleServer.createHTTPS(assetsPath, HTTPS_PORT);