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This class provides Chrome-specific stability metrics, enabling moving code
related to these metrics out of MetricsLog and MetricsService. It also
eliminates all notification listening from MetricsService. As part of the
latter change, ThreadWatcherObserver now registers explicitly for the
notifications it is concerned with rather than going through MetricsService for
this purpose.

BUG=375237

Review URL: https://codereview.chromium.org/289283011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272426 0039d316-1c4b-4281-b951-d872f2087c98