Expose ImageData on WorkerGlobalScope.

Follow the spec and also provide the ImageData interface object on
WorkerGlobalScope.

R=junov@chromium.org,eseidel@chromium.org
BUG=355030

Review URL: https://codereview.chromium.org/216923004

git-svn-id: svn://svn.chromium.org/blink/trunk@170665 bbb929c8-8fbe-4397-9dbb-9b2b20218538