Create a process-global MetadataRecorder for the stack sampling profiler

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}
3 files changed