Implement newly designed sign-in related histograms for desktop platorms.

This CL splits signin_metrics::Source into signin_metrics::AccessPoint and signin_metrics::Reason. It implements three new histograms Signin.SigninStartedAccessPoint, Signin.SigninCompletedAccessPoint and Signin.SigninReason. These histograms could replace Signin.SigninSource histogram with extra capabilities.

https://docs.google.com/a/google.com/document/d/1-gXYAMXXgsJhk6jxO55RuYJ00JBGermevJZ0sIlk6ko/edit?usp=sharingusp=sharing contains details.

BUG=532557

Review URL: https://codereview.chromium.org/1473543002

Cr-Commit-Position: refs/heads/master@{#363640}
73 files changed