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This CL removes StackCopier::Delegate::OnThreadResume and manually resets
MetadataProvider in StackSamplerImpl::RecordStackFrames, and enables
RecordMetadata() on android (more specifically StackCopiedSignal).

|read_lock_| is only acquired:
- before CopyStack() on the sampling thread. Since there is only 1 sampling
  thread per process, this won't have any contention.
- in TryReclaimInactiveSlots() with |read_lock_.Try()|. Failing to acquire
  delays ReclaimInactiveSlots().
Since read_lock_ is only held during stack capture (compared to stack walking
which takes ~3x as long), starvation is unlikely to affect TryReclaimInactiveSlots().

Bug: 1056283
Change-Id: I4f0ef3e8c3641ec7b7258da519c3fba519fb096b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2154738
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763420}