[Android] Fix build errors that are introduced by rebasing to version 31.

Some changes that have been made in upstream version 31 break XWalk for Android.
Fix the break in XWalk, including:
* Replacing AndroidBrowserProcess to BrowserStartupController.
* Changes in DevToolsServer/JavaScriptDialogManager.
* Some namespace changes.
* Removing override method ContentVideoViewClient.keepScreenOn as interface has changed.
* Removing kEnableExperimentalWebGL switch since this switch is no longer used from
  version 31 for Android port.

BUG=
TEST=Build
Merge pull request #824 from sqliu/rebase-for-31

[Android] Fix build errors that are introduced by rebasing to version 31