GitLab

Turns MetricsLogChromeOS into a metrics::MetricsProvider. Splits the
ChromeOS-specific unittests out from the MetricsLog test into separate
ChromeOSMetricsProvider tests. Also moves LogChromeOSCrash() from
MetricsService to ChromeOSMetricsProvider.

BUG=374221
R=asvitkine@chromium.org
TBR=derat, pam

Review URL: https://codereview.chromium.org/292433015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272708 0039d316-1c4b-4281-b951-d872f2087c98