Enable build flag and modify flag description for dark mode

Bug: 936275
Change-Id: Icb58cda11379b0bc5f149aafbd84bacbc8a015c2
Reviewed-on: https://chromium-review.googlesource.com/c/1493071
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Becky Zhou <huayinz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636480}
2 files changed