Fix for renderer crash during video playback

Fix for renderer crash during video playback

If we scroll/zoom the page during video playback, renderer
crash is observed. Currently, invalidation of video layer
happens on the main thread in WebMediaPlayerTizen. It should
happen on the compositor thread instead. This patch ensures
that and thus avoids the crash.

Bug: http://107.108.218.239/bugzilla/show_bug.cgi?id=11278
Reviewed by: SeungSeop Park, Venugopal S M

Change-Id: I40b7fa90c467e80af06f8ce39de4d3a763f1117e
Signed-off-by: Chandan Padhi <c.padhi@samsung.com>