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This paves the way for Chromium code to manipulate the metadata
recorder and help provide the stack sampling profiler with context
about what's happening when a sample is taken.

Bug: 913570
Change-Id: I96d7bcc2f4872467852042495c0fdf1ad5d8c4cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1572370
Commit-Queue: Charlie Andrews <charliea@chromium.org>
Commit-Queue: Mike Wittman <wittman@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Auto-Submit: Charlie Andrews <charliea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652161}