Use correct Request Context when EMBED or OBJECT requests an image

When an OBJECT or EMBED element requests an image, it does so using
an ImageLoader. To ensure that Content-Security-Policy restrictions
are applied correctly in this scenario, we must adjust the request's
context to indicate the originating element.

Bug: 811691
Change-Id: I0fd8010970a12e68e845a54310695acc0b3f7625
Reviewed-on: https://chromium-review.googlesource.com/924589
Commit-Queue: Eric Lawrence <elawrence@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537846}