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This CL extracts the core stack copying parts of
tracing::StackUnwinderAndroid into the dedicated base::StackCopierSignal
class, and adapts it for direct use by the sampling profiler stack
unwinding implementation. This drops some unneeded code; updates the code
to run on 32-bit Android, 64-bit Android, and 64-bit Linux platforms;
and converts the code to use C++ atomics.

The StackUnwinderAndroid implementation will eventually be replaced
wholesale by the sampling profiler implementation for Android. This CL
does not attempt to reuse the extracted stack copying implementation on
its own, in order to avoid introducing bugs in StackUnwinderAndroid due
to the differing implementations.

TBR=gab@chromium.org

Bug: 988579
Change-Id: I0293771679c5deae69fdbb007d8828ead575ee0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872967Reviewed-by: Mike Wittman <wittman@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Charlie Andrews <charliea@chromium.org>
Reviewed-by: ssid <ssid@chromium.org>
Commit-Queue: Mike Wittman <wittman@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709597}