Rust is required to build Chromium M121

[Dana Jansens]

Hello,

As of the M121 milestone, Rust is now part of the Chromium build in a way that can not be disabled, and the C++ code paths that were replaced by Rust will be removed in the near future.

This means the following flags are no longer supported in Chromium:

- checkout_rust: False in gclient

- enable_rust = false in GN

The GN flag will continue to exist into the future for other downstream projects that make use of our //build directory but which do not need to use Rust, however Chromium will no longer build with it disabled.

Cheers,

Dana for the Chrome Rust team.