Allow eviction of ImageBitmaps that are created from ImageElements.

We hold a reference to the HTMLImageElement in the ImageBitmap but elevate the priority of 
its CachedResource. Cropping is handled at drawing time.

Some style changes are also being introduced in this patch.
 
BUG=259947, 166658

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=155546

Review URL: https://chromiumcodereview.appspot.com/19393004

git-svn-id: svn://svn.chromium.org/blink/trunk@155629 bbb929c8-8fbe-4397-9dbb-9b2b20218538