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We observed an issue in Microsoft Edge where a certain printer driver
was being called on two different threads simultaneously and reacted
by making an unbalanced CoUninitialize() call on the browser UI thread.
If this happens enough times, the CoInitialize() count of the UI thread
will eventually drop to zero and cause problems when, for example,
out-of-process accessibility clients attempt to access objects that are
only safe to call on the UI thread - the COM marshaler will see that
the UI thread is in the implicit MTA and assume that these objects can
be called from any MTA thread. I haven't successfully reproduced the
same issue in Chrome, but the threading pattern for printer driver
calls is the same, so the potential for it to occur is there.

Existing code comments document that some Windows printer drivers are
only safe to call from the UI thread. Accordingly, this CL addresses
the issue by moving work done by the local printer handler from a
generic task runner thread to the browser UI thread on Windows.

Bug: 1065145
Change-Id: Ic20872cb33bfb6c94381059578c2055cb1a9a1eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2135054
Commit-Queue: Kevin Babbitt <kbabbitt@microsoft.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756356}