Merge pull request #24204 from davidflanagan/bug1068986

Bug 1068986: convert MediaFrame to use background-image instead of <img> r=justindarc(cherry picked from commit 5baa82f4abae9ce0d10c92a9ecee8fbc4da74e98)

Conflicts:

	shared/js/media/media_frame.js
	tests/python/gaia-ui-tests/gaiatest/apps/gallery/regions/fullscreen_image.py