Factor ChromeOS specific code out of MetricsLog.

This patch moves ChromeOS specific logging from MetricsLog
to MetricsLogChromeOS.

BUG=339100

Review URL: https://codereview.chromium.org/146913005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248588 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248588