Clean up image fallback content configuration

Put the setup of the broken image and placeholder elements in a helper
class to make it easier to decipher what is being done, and also what
depends on the ComputedStyle object and not.

Bug: 953707
Change-Id: I675539611e5160e4e31523a1dbcea830fe5c7f18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604267
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Auto-Submit: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658662}
1 file changed