Gate the caption settings button behind the kLiveTranslate flag

This CL puts the caption settings button in the global media control behind the kLiveTranslate flag. This new UX will be rolled out as a part of the Live Translate launch.

Bug: 1426899
Change-Id: I568908905ac3532c4ed4e0546b9245045b4b0934
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4568846


Reviewed-by: Tommy Steimel <steimel@chromium.org>
Commit-Queue: Evan Liu <evliu@google.com>
Cr-Commit-Position: refs/heads/main@{#1149447}