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When clearing the contents of a framebuffer, also invalidate/discard it
if the GL implementation supports this functionality. This improves
performance especially on tiling architecture GPUs since the graphics
driver does not need to restore the previous contents of the framebuffer
before proceeding with rendering.

Note that normally we don't clear the contents of framebuffers whose
contents we know we will completely re-render. This patch will still
invalidate such framebuffers to let the driver know that the previous
contents are not relevant.

BUG=274334

Review URL: https://chromiumcodereview.appspot.com/23601013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221425 0039d316-1c4b-4281-b951-d872f2087c98