[sms] Create a new chrome://flag to decouple the desktop integration

We want to allow the development of the desktop integration while
not affecting our origin trial users. We introduce a new chrome://flag
which needs to be turned on explicitly while we are still working
on the UX/privacy/security implications.

Bug: 1015645
Change-Id: I6b13bfb261ef96b8582ff23f7c00756a0e921bc4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1925238
Commit-Queue: Sam Goto <[email protected]>
Reviewed-by: Reilly Grant <[email protected]>
Reviewed-by: Alex Chau <[email protected]>
Cr-Commit-Position: refs/heads/master@{#717346}
10 files changed