Implement all sign in flows with embedded webview

a first version with known limitations:
1. does not handle error cases
2. does not support SAML case
3. does not prefill user email based on the main cookie jar

BUG=

Review URL: https://codereview.chromium.org/26334005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230007 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230007