Merge "ImageInputType::ensurePrimaryContent should recreate UA shadow tree." to M49 branch

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}
(cherry picked from commit 2386a6a49ea992a1e859eb0296c1cc53e5772cdb)

Review URL: https://codereview.chromium.org/1756363003 .

Cr-Commit-Position: refs/branch-heads/2623@{#566}
Cr-Branched-From: 92d77538a86529ca35f9220bd3cd512cbea1f086-refs/heads/master@{#369907}
2 files changed