merge_pr_62898

[webauthn] Add remoteClientDataJSON renderer surface

Add the complete renderer-side input and output surface for the remoteClientDataJSON WebAuthn extension. The preceding CLs provide the Mojo contract and browser-side input/output handling.

• Add the experimental WebAuthenticationRemoteClientDataJson runtime feature and map device::kWebAuthnRemoteClientDataJson to it.
• Add remoteClientDataJSON to the input, FromJSON, output, and output-JSON IDL dictionaries behind the runtime feature.
• Convert renderer input values to Mojo and browser output values back to the IDL boolean result.
• Surface the create-flow output through getClientExtensionResults().
• Advertise extension:remoteClientDataJSON from getClientCapabilities().
• Reject incompatible conditional and immediate mediation combinations at the renderer boundary.
• Cover input conversion, JSON round trips, mediation rejection, and extension output conversion.

AI-Assisted Development: developed with assistance from Anthropic's Claude (Claude Code CLI).

Bug: 506062130
Change-Id: Ie2a65b4a5153894ab4e670d10507479dde6578c1
Spec: https://w3c.github.io/webauthn/#sctn-remote-client-data-json-extension
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8177083
Reviewed-by: Ken Buchanan kenrb@chromium.org
Reviewed-by: Martin Kreichgauer martinkr@google.com
Commit-Queue: Ken Buchanan kenrb@chromium.org
Reviewed-by: Rick Byers rbyers@chromium.org
Cr-Commit-Position: refs/heads/main@{#1703589}