Making canvas elements declare their external memory consumption to V8.

BUG=265594
TEST=LayoutTest/fast/canvas/canvas-large-temporary.html

Review URL: https://chromiumcodereview.appspot.com/21145005

git-svn-id: svn://svn.chromium.org/blink/trunk@155193 bbb929c8-8fbe-4397-9dbb-9b2b20218538