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This CL changes SharedRemote so that its sync calls now block only the
calling sequence rather than additionally blocking the underlying
Remote's bound sequence.

Fixed: 1016022
Change-Id: I2483c130d882de24686c0bb1396a894171a733fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1871249
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Darin Fisher <darin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708028}