GPU process SwapBuffers from child to parent context calls glFlush before invoking SwapBuffers callback.
GPU process SwapBuffers from child to parent context calls glFlush before invoking SwapBuffers callback.

This is required for the side effects of the swap to be guaranteed visible to teh parent context.

Also fixed the back buffer clearing to work with ANGLE.

TEST=try, change the default clear color to test that the back buffer was actually cleared.
BUG=none

Review URL: http://codereview.chromium.org/3170035

git-svn-id: http://src.chromium.org/svn/trunk/src@58413 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: ef721365c1bbe9320ae4bb1be8b1d148eda4e280
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser