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The CFI unwinder only works for chrome stack frames. Write a hybrid
unwinder which can use libunwind for android and CFI unwinder for
chrome. Does not support unwinding in x86 and arm 64bit builds.
It supports pausing a different thread and unwinding frames on that
thread.

This will be useful for UMA sampling profiler and tracing profiler.

BUG=859260

Change-Id: I208b9b21c8f6a73a4d593f1446095da92c0cb848
Reviewed-on: https://chromium-review.googlesource.com/1132144
Commit-Queue: Siddhartha S <ssid@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Reviewed-by: David Turner <digit@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579598}