Add a raw-pixel based SkBitmap as a cache storage at CPU side for WebGLImageBufferSurface.
It can avoid heavy cost on redundant readback from GPU side to CPU side.

BUG=328276

Review URL: https://codereview.chromium.org/107903003

git-svn-id: svn://svn.chromium.org/blink/trunk@164249 bbb929c8-8fbe-4397-9dbb-9b2b20218538