[scheduler] Changing the UiThreadTaskTraits API to be static.

As part of the ongoing process of unifying Java and C++ scheduling, I am
changing the UiThreadTaskTraits API to be static. This will be useful in
migrating the callers of ThreadUtils.postOnUiThread to PostTask.postTask
(they won't have to allocate an instance of UiThreadTaskTraits) and be
more in compliance with the content_public policy.

Bug: 863341
Change-Id: I1b8a96889a3ce087a1937e96586b8bf2820b8f4f
Reviewed-on: https://chromium-review.googlesource.com/c/1426693
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Karolina Soltys <ksolt@chromium.org>
Auto-Submit: Karolina Soltys <ksolt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627883}
6 files changed