[Experiment] Remove FedCM from Cobalt

[linxinan-yt]

Bug: 505811196

[github-actions]

🤖 Gemini Suggested Commit Message

---

cobalt: Disable Privacy Sandbox APIs by default

Disable Privacy Sandbox APIs by setting the default build argument to
false and adding preprocessor guards around associated components.
This change ensures that features like Federated Credential Management
(FedCM), Fenced Frames, and Trust Tokens are excluded from the build.

Excluding these components reduces the Cobalt binary size and removes
APIs that are not required for the current target platforms. Logic
for Credential Management in Blink and Trust Tokens in the network
service is now conditionally compiled based on the build flag.

Bug: 505811196

---

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[gemini-code-assist]

Code Review

This pull request disables the Privacy Sandbox APIs by setting enable_privacy_sandbox_apis to false in cobalt.gni and conditionally gating related features (such as Fenced Frames, WebID/FedCM, Trust Tokens, and Secure Payment Confirmation) across content/, services/network/, and third_party/blink/ using preprocessor guards and build configuration filters. Feedback is provided regarding an incomplete comment in render_frame_host_impl.cc where a line was accidentally deleted during refactoring.

[gemini-code-assist]

The second line of the comment was accidentally deleted during refactoring, leaving the sentence incomplete. Please restore the full comment.

  // An ancestor's network revocation status could've changed as a result of
  // this fenced frame being removed.