Adopt new rust_bindgen behaviour in Rust mojo impl

The rust_bindgen template now makes a library that exports the bindgen
generated symbols.

R=lukasza@chromium.org

Bug: b/373864033
Cq-Include-Trybots: luci.chromium.try:android-rust-arm64-rel,android-rust-arm32-rel,android-rust-arm64-dbg,linux-rust-x64-dbg,linux-rust-x64-rel,mac-rust-x64-dbg,win-rust-x64-dbg,win-rust-x64-rel
Change-Id: I4584b004907612a3c122f4cf39dddc397288332a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5973313
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1374768}


CrOS-Libchrome-Original-Commit: 09afac1d1aa18250073002ddedd0e064e4c2a981

Author: danakj

mojo/public/rust/system/ffi.rs

@@ -17,12 +17,8 @@
 //! TODO(crbug.com/40206847):
 //! * Remove references to the now-nonexistent mojo Github
 
-pub mod raw_ffi {
-    #![allow(dead_code)]
-    #![allow(non_upper_case_globals)]
-    #![allow(non_camel_case_types)]
-    #![allow(non_snake_case)]
-    include!(env!("BINDGEN_RS_FILE"));
+chromium::import! {
+  pub "//mojo/public/rust:mojo_c_system_binding" as raw_ffi;
 }
 
 pub mod types {