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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.

The reland enables the StackCopierSignalTest.CopyStackFromOtherThread
test on 32-bit Android only.

TBR=gab@chromium.org

Bug: 988579, 1018615
Change-Id: I96704fe4843994721a0eb37896e11bea99be3fda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884376
Commit-Queue: Mike Wittman <wittman@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Charlie Andrews <charliea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710415}