Add a feature flag for send_tab_to_self::kSendTabToSelfWhenSignedIn

This will gate near-term-experimental work in making self-share-tabs
available to signed-in but not-syncing users.

Bug: 956137, 956722

Change-Id: I0a1177d6437d77376aa52a111217f21446a7afd7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1579759
Commit-Queue: Travis Skare <skare@chromium.org>
Reviewed-by: sebsg <sebsg@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654216}
7 files changed