preemptive add new virtual for onDrawPicture

skia is changing the signature for onDrawPicture, so this CL adds the new one, and removes OVERRIDE from the old.

BUG=

Review URL: https://codereview.chromium.org/454993002

git-svn-id: svn://svn.chromium.org/blink/trunk@179885 bbb929c8-8fbe-4397-9dbb-9b2b20218538