Add libprotobuf-mutator to DEPS

Required for GN generation when fuzzing is enabled (use_libfuzzer=true).
Recent Chromium changes (http://crrev.com/c/7988920) made Well-Known
Type protos transitively depend on it.

Bug: None
Change-Id: Iea2dfb4f98e9eed9a88c3768b12d59f226b7fbab
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/485720
Commit-Queue: Johannes Kron <kron@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48092}

DEPS

diff --git a/DEPS b/DEPS
index afbe821..40e0f10 100644
--- a/DEPS
+++ b/DEPS
@@ -606,6 +606,8 @@
     'https://chromium.googlesource.com/external/github.com/llvm/llvm-project/compiler-rt/lib/fuzzer.git@4b46c0ed586f91aa1ad9b5ebbcad907c9dd956e5',
   'src/third_party/fuzztest/src':
     'https://chromium.googlesource.com/external/github.com/google/fuzztest.git@6d428b093eb95758ec27c92026b7590b51cbaad0',
+  'src/third_party/libprotobuf-mutator/src':
+    Var('chromium_git') + '/external/github.com/google/libprotobuf-mutator.git@c1c950eae0440c3808f2b8bd7c57d0c6a42c1a90',
   'src/third_party/libjpeg_turbo':
     'https://chromium.googlesource.com/chromium/deps/libjpeg_turbo.git@640f254ad0fa03f6b1f29f89b7dd9366f2f6e533',
   'src/third_party/libsrtp':
@@ -1395,4 +1397,4 @@
     "+modules/audio_device",
     "+modules/audio_processing",
   ]
-}
\ No newline at end of file
+}