Make OffscreenCanvas resizeable

This change implements all the script observable aspects of changing
the size of an OffscreenCanvas object.  What is missing after this
change is to correctly handle the compositing layer updates for the
commit() flow.

BUG=662498

Review-Url: https://codereview.chromium.org/2508943003
Cr-Commit-Position: refs/heads/master@{#432881}
14 files changed