Identity API: Add AccountTracker and attach it to identity.onSignInChanged

The AccountTracker class fetches gaia IDs for all accounts signed into
Chrome, and calls its observers any time an account is added, removed, or
changes sign-in state.

The IdentityAPI class observes the AccountTracker and routes the events
to listening apps and extensions.

BUG=305830

Review URL: https://codereview.chromium.org/30503002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230017 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230017
Add missing #includes of <algorithm> in ppapi for std::min/max

Required when building on VS2013.

TBR=dmichael@chromium.org
BUG=288948

Review URL: https://codereview.chromium.org/32343005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230018 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230018