Experiment with USER_VISIBLE priority for update_jumplisticons_task_runner_

It is preferable to use BACKGROUND priority for jumplist update thread
as it is now. This CL is to see how long it takes for jumplist update if
we assign the thread USER_VISIBLE priority.

BUG=40407, 179576

Review-Url: https://codereview.chromium.org/2811003002
Cr-Commit-Position: refs/heads/master@{#463812}
4 files changed