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This fixes some edge cases which can cause SharedRemote to either block
unnecessarily on its bound sequence or fail to block as intended on its
bound sequence, depending on how it's constructed and called into.

We want to repurpose the underlying Remote's sync waiting behavior when
the SharedRemote makes a sync call from the bound sequence, but when a
call is made from any other sequence, the SharedRemote must implement
its own waiting mechanism.

This change ensures that SharedRemote configures its underlying Remote
to disable sync waiting by default, and it's selectively re-enabled only
around any (potentially nested) sync calls made from the bound sequence.

Fixed: 1102921
Change-Id: I603e48e38d859c43d7b732a91445314714ea0591
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2286051
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786180}