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To allow construction with multiple unwinder. This is necessary on android
to provide native and chrome unwinders and will be used in a follow in
stack_sampling_profiler_unittest.cc
https://chromium-review.googlesource.com/c/chromium/src/+/2201658

Bug: 989102
Change-Id: I05ef97403668d82dc7e6e4e9505be95e941174dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2211862
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773353}