Stop OffscreenCanvas resize from changing placeholder attributes

For consistency with ImageBitmapRenderingContext, the content
width and height attributes of a placeholder canvas should simply
have no effect, nor should they reflect the intrinsic size of the
canvas.

BUG=662498

Review-Url: https://codereview.chromium.org/2515623003
Cr-Commit-Position: refs/heads/master@{#433299}
6 files changed