gpu: Disable EXT_discard_framebuffer on Chrome OS

Disable the EXT_discard_framebuffer extension on Chrome OS.
It causes flickering and jumpy scrolling.

BUG=301988
TEST=Open www.monitor.hr on Daisy and scroll up and down

Signed-off-by: Dominik Behr <dbehr@chromium.org>

Review URL: https://codereview.chromium.org/26758002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228822 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228822