[Android] Fix the black screen issue of fullscreen video playback introduced by rebase.

In 35, new method added for the fullscreen video playback, this requires a new method to
be called before enter/exit fullscreen.  An public method added into XWalkView for the
method call.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1431
TEST=
Merge pull request #1827 from sqliu/video_fullscreen

[Android] Fix the black screen issue of fullscreen video playback introd...