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Refactors the cross-platform parts of the stack sampling into
the new StackSamplerImpl implementation of the StackSampler
interface. Platform-specific aspects of stack sampling are refactored
into the ThreadDelegate/ThreadDelegateWin interface/implementation.

Refactoring the Mac sampler to make use of StackSamplerImpl will be
addressed in the next change.

Bug: 931418
Change-Id: I204257fec31b23fc20c7beceddb81b3a38a9466d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1530009
Auto-Submit: Mike Wittman <wittman@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Charlie Andrews <charliea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644448}