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The COM objects participating in the on-demand transactions on the
server side are free threaded. That means that they can be entered
by any COM RPC threads when calls  on their interfaces are made by
the on-demand client on Chrome.

These objects can also be entered by the thread pool threads of
the updater, when callbacks from the update service are posted.

This CL introduces a sequential task runner to `move` the callbacks
which occur in the main sequence onto a new sequence, which can block on
base synchronization primitives. Then, it introduces a lock to
serialize access to the free threaded COM objects.

BUG:1074027

Change-Id: Icc8f579aebf943e5d43965f6f7d2a63e664bf0bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2161347
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Reviewed-by: S. Ganesh <ganesh@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762461}