Feature flag to prioritize start up related tasks

Some of the tasks are posted before native is initialized. In order to
read the flag value before native is initialized we make use of
FeatureUtilities.

This patch just adds the flag, there will be many follow ups making
actual use of it.

Original patch by Carlos Caballero <carlscab@google.com>.

Bug: 863341
Change-Id: If95542568c9074b9031250fb7c702aaf033114c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1503416
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Yaron Friedman <yfriedman@chromium.org>
Auto-Submit: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638210}
6 files changed