GitLab

Creation of a new signin_metrics class to track components/signin UMA.

BUG=357693
TEST=New histograms are called Profile.NumberOfAccountsPerProfile,
Signin.ReconcilerAddedToChrome and Signin.ReconcilerAddedToCookieJar.
There is also a UserCount called
AccountReconcilerDifferentPrimaryAccounts. These can be viewed in
chrome://histograms and chrome://user-actions respectively.
The Profile one can be viewed any time a profile is opened. The others
are best tested the first time you Sign In to Chrome, assuming you
already were signed in to google in the content area with 1 or more
accounts.

Review URL: https://codereview.chromium.org/270673003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269789 0039d316-1c4b-4281-b951-d872f2087c98