CSRF tokens should only be required (and only be allowed) if
the current session isn't already inherently safe against
CSRF due to the way the authentication mechanism works.
This allows (and requires) tokens to be omitted for requests
that use an OAuth Authorization header.
Bug: T230843
Bug: T230842
Bug: T237852
Change-Id: Ib2922d556ff2470d4bf8c386c18986ca9f37d1b5
NOTE: once this is merged, also merge Ie7b47e6868cc on the OAuth repo,
to fix unit tests after a breaking change to Router's constructor
signature.
Bug: T230842
Change-Id: I8f5b92918a58e44a4f2d8c78d234d9f64c2d06bf