[java] Prioritize bootstrap and navigation tasks behind a feature flag.

When prioritizing tasks SingleThreadTaskRunnerImpl will post async
messages at the head of the queue.  It's important for these messages to
be async because we want to bypass the java fences and run the tasks
immediately, rather than having them batched up to run every vsync.

Bug: 863341
Change-Id: I4faf7846e628cc05c05809e626d22788ae9a148c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1514533
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641132}
5 files changed