Add feature flag for web contents dark mode

Bug: 933492
Change-Id: I48fc19c185ebb0581045668d07e057fb12130521
Reviewed-on: https://chromium-review.googlesource.com/c/1478106
Commit-Queue: Becky Zhou <huayinz@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633748}
12 files changed