Add tasks traits for BrowserUIThreadScheduler

Add a new task trait to tell the future BrowserUIThreadScheduler which
tasks are critical for startup and allow it to prioritize them
accordingly.

Design doc: https://docs.google.com/document/d/1z1BDq9vzcEpkhN9LSPF5XMnZ0kLJ8mWWkNAi4OI7cos/edit#

Bug: 863341, 872372
Change-Id: Ifb082762726fbda951325802faf82f1dacf219cc
Reviewed-on: https://chromium-review.googlesource.com/c/1454632
Commit-Queue: Carlos Caballero <carlscab@google.com>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629944}
4 files changed