ImageInputType::ensurePrimaryContent should recreate UA shadow tree.

Once the fallback shadow tree was created, it was never recreated even if
ensurePrimaryContent was called.  Such situation happens by updating |src|
attribute.

BUG=589838

Review URL: https://codereview.chromium.org/1732753004

Cr-Commit-Position: refs/heads/master@{#377804}