Remove image_decode_tasks_enabled flag.

This flag should be on by default in all non-test cases, so this patch
removes the flag altogether.

BUG=696864
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2781183002
Cr-Commit-Position: refs/heads/master@{#467725}
20 files changed