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Refactors the Mac sampling to use the StackSamplerImpl implementation,
with the platform-specific aspects moved into ThreadDelegateMac.

Bug: 931418
Change-Id: I8f468fa09b9ddc85921f21563ff41000a5c968f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1535178Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644804}