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ServiceManager may need to launch child processes before a BrowserMainLoop
has been created, and therefore before any BrowserThreads exists. We
therefore replace the BrowserThread::PROCESS_LAUNCHER with a global
TaskScheduler sequence, created on-demand when the ServiceManager or
ChildProcessLauncherHelper first need it.

Under Windows we must use a single-thread TaskRunner, while on other
platforms we are able to use a normal TaskScheduler sequence. File
crbug.com/820200 to track that.

Bug: 815225
Change-Id: Ia0f46461fb9cc92fddacf81ee96b764de8477d11
Reviewed-on: https://chromium-review.googlesource.com/941264
Commit-Queue: Xi Han <hanxi@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542212}