GitLab

According to the latest data, about 0.02% of DB initialization results in discarding old DB which may contain users' unsynced data.
About 2/3 of these failures are due to errors detected by CheckValidity().

Add UMA histogram to help future developers improve this.

BUG=None

Review URL: https://codereview.chromium.org/445513002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288010 0039d316-1c4b-4281-b951-d872f2087c98