Verify cargo vet cleaniness in third_party/rust/PRESUBMIT.py.

Example presubmit failure (after manually, temporarily deleting parts of
`//third_party/rust/chromium_crates_io/supply-chain/audits.toml`):

```
$ git cl presubmit --force
Running Python 3 presubmit commit checks ...
  checking owners took a long time: 0.6s
  checking for commit objects in tree took a long time: 0.6s
** Presubmit Warnings: 1 **
//tools/crates/run_cargo_vet.py check
vpython3 /usr/local/google/home/lukasza/src/chromium2/src/third_party/rust/../../tools/crates/run_cargo_vet.py check --locked --frozen --no-minimize-exemptions (0.30s) failed
Vetting Failed!

2 unvetted dependencies:
  unicode-width:0.1.11 missing ["safe-to-run", "does-not-implement-crypto"]
  wycheproof:0.4.0 missing ["safe-to-run", "does-not-implement-crypto"]
Running cargo -Zunstable-options -C /usr/local/google/home/lukasza/src/chromium2/src/tools/crates/../../third_party/rust/chromium_crates_io vet check --locked --frozen --no-minimize-exemptions --cargo-arg=-Zbindeps --no-registry-suggestions
Failed.


Presubmit checks took 5.3s to calculate.
There were Python 3 presubmit warnings.
```

Bug: 326234536
Change-Id: I09040d6dce8900b43a7002cad11439572432feae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5388131
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1277673}

docs/rust-unsafe.md

@@ -54,9 +54,4 @@ Additional notes:
   (e.g. Android, Fuchsia, etc.).  This means that adding a new crate does not
   necessarily require a new audit if the crate has already been audited by
   other projects.
-* For now we kindly ask you that audit updates in `audits.toml`
-  should be part of the submitted CL so that `run_cargo_vet.py check` will
-  continue to pass after the CL lands.
-    - TODO(https://crbug.com/326234536): Enforce `cargo vet` cleaniness via
-      `PRESUBMIT.py`.
 

third_party/rust/PRESUBMIT.py

@@ -0,0 +1,31 @@
+# Copyright 2024 The Chromium Authors
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Top-level presubmit for //third_party/rust
+
+See https://www.chromium.org/developers/how-tos/depottools/presubmit-scripts
+for more details about the presubmit API built into depot_tools.
+"""
+
+PRESUBMIT_VERSION = '2.0.0'
+
+def CheckCargoVet(input_api, output_api):
+    vet_args = ['check']
+
+    # Hermetic and idempotent.
+    vet_args += ['--locked', '--frozen', '--no-minimize-exemptions']
+
+    run_cargo_vet_path = input_api.os_path.join(
+        input_api.PresubmitLocalPath(),
+        '..', '..', 'tools', 'crates', 'run_cargo_vet.py')
+    cmd_name = '//tools/crates/run_cargo_vet.py check'
+    test_cmd = input_api.Command(
+        name=cmd_name,
+        cmd=[input_api.python3_executable, run_cargo_vet_path] + vet_args,
+        kwargs={},
+        message=output_api.PresubmitPromptWarning)
+    if input_api.verbose:
+        print('Running ' + cmd_name)
+    return input_api.RunTests([test_cmd])
+