net/: Use both QUIC and TCP for initial URL requests

[ckim1346]

This PR reintroduces the use_quic_for_unknown_origins flag to try both QUIC and TCP for initial/unknown URL requests. If QUIC returns first, it will cancel the TCP request and otherwise fall back on the TCP ensuring we try quick but also have a reliable fallback. Additionally, we allow non-production builds to use any port whereas production builds can only use port 443.

Please refer to #2981 and #3514.

Bug: 467778593

[github-actions]

🤖 Gemini Suggested Commit Message

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net: Race QUIC/TCP connections for Android TV

Introduce a flag to enable racing QUIC and TCP connections for initial
URL requests to origins without established Alternative Service records.
This mechanism allows the system to concurrently attempt connections
using both protocols and utilize the one that connects faster.

This optimization improves initial connection setup latency, especially
for first-time connections to a server that supports QUIC but has not
yet advertised it via Alt-Svc. This feature is enabled specifically for
Android TV builds.

Bug: 467778593

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[johnxwork]

When would port not be 443?

I also think it's dangerous to have qa and gold differ on this behavior. Prod could break if there's some issue with the prod-specific code.

[ckim1346]

#3514 I would assume for testing/debugging purposes -- @kaidokert do you remember the exact reasoning?

[jellefoks]

I think this is for mirroring the enforcing of https for gold builds.

[jellefoks]

I think we should at least enforce it to be a privileged port (<1024).

[gemini-code-assist]

Code Review

This pull request reintroduces the use_quic_for_unknown_origins feature, which opportunistically attempts a QUIC connection for initial requests to improve performance, with a fallback to TCP. The implementation correctly restricts this behavior to port 443 for production builds. However, there is a critical preprocessor logic error in http_stream_factory_job_controller.cc that will cause a compilation failure. I've provided a comment with a suggested fix for this issue.