Convert OptionalShareTargetsManager to use a SequencedTaskRunner.

The previous implementation would wait on async task and on certain
device that could result in a deadlock.  This converts to the new
task queuing system, which avoids the need for waiting.

BUG=949053

Change-Id: I795df73741e5d8e14255255a81aad6fd7eda6cf1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715568
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Commit-Queue: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681326}