Leverage glDiscardFramebuffer for tiled-based GPU to improve preserveDrawingBuffer=false WebGL
performance and also save memory bandwidth. In case of glDiscardFramebuffer, there is no need
to call glClear again.

BUG=418272

Review URL: https://codereview.chromium.org/614883002

git-svn-id: svn://svn.chromium.org/blink/trunk@183217 bbb929c8-8fbe-4397-9dbb-9b2b20218538