WebGL: Fix not-clear of non-preserveDrawingBuffer.

Current WebGL relies on glDiscardFramebufferExt() to clear FBO, but the API
doesn't guarantee clearing FBO.

TEST=fast/canvas/webgl/draw-webgl-to-canvas-2d.html,
fast/canvas/webgl/webgl-composite-modes-repaint.html
BUG=423674

Review URL: https://codereview.chromium.org/656053002

git-svn-id: svn://svn.chromium.org/blink/trunk@183804 bbb929c8-8fbe-4397-9dbb-9b2b20218538