Display access token information on about:signin-internal page.

This change adds interfaces to OAuth2TokenService::Observer for observing access token request events. The token details section on about:signin-internal is repurposed to display account, token requester, request time and request status of access tokens.

BUG=309106,311414
TBR=benwells@chromium.org,brettw@chromium.org,rlarocque@chromium.org,gene@chromium.org,bauerb@chromium.org,ajwong@chromium.org,zea@chromium.org,ben@chromium.org,mnissler@chromium.org,nkostylev@chromium.org

Review URL: https://codereview.chromium.org/64853006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242580 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242580