GitLab

For normal images contentAttributes are optional in
the spec but the converter requires them. This makes
them optional.

Change-Id: I4073de51009e6e7802667b2d79a7ad485d939978
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363477
Auto-Submit: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Sam Bowen <sgbowen@google.com>
Reviewed-by: Sam Bowen <sgbowen@google.com>
Cr-Commit-Position: refs/heads/master@{#799388}