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This interface will be implemented in following CLs and used in
StackSamplerImpl. The main thrust of this change is to move utility
functions used by the stack copying (and their tests) to their
ultimate location.

No functional change is intended.

TBR=gab@chromium.org

Bug: 988579
Change-Id: I45d8c5ca155dd16dd69875c01548dd5e1ab9602d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1744907Reviewed-by: Mike Wittman <wittman@chromium.org>
Reviewed-by: Charlie Andrews <charliea@chromium.org>
Commit-Queue: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691820}