Explicitly flush the underlying SkDeferredCanvas for HW accelerated 2D canvas when drawImage from WebGL.

When drawing a TextureBacked SkBitmap, it should draw immediately instead of deferring it for later batch draw to avoid
the situation that the TextureBacked SkBitmap is changed before the actual draw and the change is refelected to the
Canvas as well.

BUG=326080

Review URL: https://codereview.chromium.org/101253002

git-svn-id: svn://svn.chromium.org/blink/trunk@163486 bbb929c8-8fbe-4397-9dbb-9b2b20218538