Update Chrome decision trees based on feedback

[pkotwicz]

I have created separate decision trees for:

• Toplevel auth code sign in.
• Embedded implicit grant silent sign in.
• Social widget.

I know that I am missing:

• Session extension
• Embedded non-silent sign in (such as third-party commenting widget like Disqus)

I suspect that I got some of the terminology wrong for the implicit grant sign in flow. The important bit is that the diagram outlines a roundabout way that the IdP iframe can get access to the IdP first party cookie if the user accepted the FedCM prompt for the (toplevel_frame_url, idp_url) pair