webkit: Manage video playing state properly when browser goes to background

Issue: Browser App Video streaming not pausing while pressing home Button

Fix : When the Browser goes to the background while the video is playing
      in the fullscreen, the fullscreen mode is exited. While exiting
      the full screen mode the video state is handled properly. So that
      there is no playback in the background.

CRs-Fixed: 509225

Change-Id: I32c306923d2638731d34d216cf006050ce4ad825