Add QueuePriority::kVeryHighPriority

We want to be able to prioritize loading tasks in blink but we need to
ensure loading control tasks have a higher priority than loading tasks
but below highest (which is used for input).  This requires us to add
yet another priority.

Bug: 863341
Change-Id: I1e2b76220d37b632b8706863288df3b323f2a6f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625111
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662271}
6 files changed