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Moves creation of Unwinders from the profiled thread to the profiler
thread on Android, Windows, and Mac. This initialization is expensive
on Android -- 50+ ms -- so it's important to not block the profiled
thread on performing it. Mac does some work in its unwinder
construction so might see a small benefit as well.

This is accomplished by passing a factory function for creating
the unwinders into StackSamplingProfiler rather than the unwinders
themselves. The naming of the classes that create the individual
unwinders in ThreadProfiler is updated to call them 'creators',
to minimize confusion over multiple levels of factories.

Also updates class-level documentation on how to use the profiler.

Bug: 1129082
Change-Id: I791538c72dce5c0d04c4f30e2f287fef96da1db6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414773
Commit-Queue: Mike Wittman <wittman@chromium.org>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: ssid <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809202}