Adding a debug setting to enable visual indicator for GL

  [This is the WebView part]
  The idea is to turn on the visual indicator without building the code.

  The implementation included:
  1. Setup the UI on browser side to check whether or not this is enabled.
  2. Transfer the info from browser setting to web setting.
  3. Send this info down from WebView to webkit.
  4. In the webkit, we save this info in TilesManager.
  5. At texture generation time, we query this info to decide whether or
     not add the visual indicator on the texture.

  One design decision we made is we don't want to restart the browser for
  debugging purpose. This is better preserving the browser current activity,
  the only pitfall is that the visual indicator is NOT updated on different
  textures simultaneously.

  The corresponding browser change is: #change,97058
  The dependent webkit change is: #change,97052

bug:3458721

Change-Id: I34a0514048df61b414c3386b292f2586efbde74e