Access printer drivers on a single thread on Windows.

Print Preview currently talk to printers through a thread pool in
LocalPrinterHandlerDefault. This includes the possibly of talking to
multiple printers simultaneously. This is fine with CUPS, but not safe
on Windows. Change LocalPrinterHandlerDefault to have a TaskRunner. For
Windows, make the TaskRunner a SingleThreadTaskRunner.

BUG=957242

Change-Id: Iad5bd30799504f2104c808057139bf06223d4128
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604206
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658397}
2 files changed