Improve the accuracy of V8 external allocation reporting in HTMLCanvasElement

This change modifies the existing V8 external allocation reporting in
HTMLCanvasElement to make it report the memory consumed by all
the image buffers that are held by the canvas element. This adresses
an issue where cyclic allocation of WebGL contexts was not putting
enough pressure on V8 to trigger garbage collections frequently
enough to prevent the browser from running out of GPU memory.

BUG=351871

Review URL: https://codereview.chromium.org/199563002

git-svn-id: svn://svn.chromium.org/blink/trunk@169294 bbb929c8-8fbe-4397-9dbb-9b2b20218538