communicate drawing pause to webkit to pause gif animation

Gif frames continue even if the webkit changes aren't drawn.
When we suspend drawing, suspend the gif animation as well.
This is accomplished by fooling webkit into thinking the
browser window is offscreen. When the webkit drawing is
resumed, invalidate the entire content so the gif animations
start up once more.

requires companion change in external/webkit

http://b/2621902

Change-Id: I1814aa41630820872d62752b75ae94383fdcc5fc