…33353

Enables support for Webauthn discoverable credentials (aka resident
credentials). This allows users to authenticate without first having to
select or type a username.

To decide if discoverable credentials are supported, the class
'AuthenticatorCommon', in the chrome content code, indirectly calls the
method 'context::WebAuthenticationDelegate.SupportsResidentKeys(..)'.
The default implementation of this returns false, leaving it up to
specific implementations to override.

This change adds a new class 'ElectronWebAuthenticationDelegate' to
subclass 'WebAuthenticationDelegate' and override the behaviour of the
'SupportsResidentKeys' method to return true.
The implementation is copied from the Chrome browser equivalent
'ChromeWebAuthenticationDelegate', though the chrome class includes
other methods that don't seem to be required for this functionality.

The 'ElectronContentClient' class was also updated to store an instance
of 'ElectronWebAuthenticationDelegate', and to provide an accessor
method, GetWebAuthenticationDelegate().

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