Redraw only dirty area for accelerated 2D Canvas.

When 2D Canvas got it's own layer, the whole canvas was getting
updated instead of only the dirty rect.

BUG=382958

Review URL: https://codereview.chromium.org/341213002

git-svn-id: svn://svn.chromium.org/blink/trunk@176815 bbb929c8-8fbe-4397-9dbb-9b2b20218538