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Adds a function to the Unwinder interface that is invoked at the time the
stack is captured, while the target thread is suspended, and wires up the
StackCopiers and StackSamplerImpl to invoke it at the appropriate time.

This function will be overridden in the new V8 unwinder to save off the
active V8 code pages while the thread is suspended.

Bug: 1035855
Change-Id: Ide287443faed211e2e6d205ebe78b8a536caa729
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2074806
Commit-Queue: Mike Wittman <wittman@chromium.org>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747393}