cc: Unittest for checking already failed image decode task.

Add unittest for checking already failed image decode task does not
get added for decoding next time.

BUG=606117
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/1933723002
Cr-Commit-Position: refs/heads/master@{#391159}
3 files changed