js: Explicitly kill Firefox on quit

The FirefoxDriver will kill the browser when it receives the quit command, but
we still need to explicitly kill the subprocess so we can be notified when the
process has died. Only then should we delete the profile directory. This should
eliminate the race condition for issue 8564.

javascript/node/selenium-webdriver/firefox/index.js

@@ -237,6 +237,9 @@ var Driver = function(opt_config, opt_flow) {
   /** @private {?string} */
   this.profilePath_ = null;
 
+  /** @private {!Binary} */
+  this.binary_ = binary;
+
   var self = this;
   var serverUrl = portprober.findFreePort().then(function(port) {
     var prepareProfile;
@@ -289,11 +292,15 @@ Driver.prototype.setFileDetector = function() {
 Driver.prototype.quit = function() {
   return this.call(function() {
     var self = this;
-    return Driver.super_.prototype.quit.call(this).thenFinally(function() {
-      if (self.profilePath_) {
-        return io.rmDir(self.profilePath_);
-      }
-    });
+    return Driver.super_.prototype.quit.call(this)
+        .thenFinally(function() {
+          return self.binary_.kill();
+        })
+        .thenFinally(function() {
+          if (self.profilePath_) {
+            return io.rmDir(self.profilePath_);
+          }
+        });
   }, this);
 };