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The Windows Thread Pool API provides a CallbackMayRunLong method to
indicate that a callback may not return quickly. The thread pool can use
this information to better determine when a new thread should be
created.

This CL teaches PlatformNativeWorkerPoolWin about ScopedBlockingCall in
order to determine whether a callback will return quickly. A
BlockingObserver is registered before running a task in the work
callback. If the task contains a ScopedBlockingCall, the observer is
notified, and will call CallbackMayRunLong on the appropriate
PTP_CALLBACK_INSTANCE.

Bug: 756547
Change-Id: If04d2c5e01b00960dc510e94ff89525036c2ae67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574229
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653252}