GitLab

- All NearbySharingService methods should run in same sequence, adding DCHECK_CALLED_ON_VALID_SEQUENCE to enforce that
- Also changed RegisterSendSurface/UnregisterSendSurface to synchronous

Bug: 1084644
Change-Id: I6eac1f5ad082144b2ff3d2c366b7855c93f42592
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2308410
Commit-Queue: Alex Chau <alexchau@chromium.org>
Reviewed-by: Richard Knoll <knollr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790330}