Explicitly pass -Wno-unused-function to the targets that need it.

third_party/brotli/BUILD.gn

Index: third_party/brotli/BUILD.gn
diff --git a/third_party/brotli/BUILD.gn b/third_party/brotli/BUILD.gn
index 8e1c111260ed150101d2049508fe6ce1e5d5f095..2975f036df401585fe627b329471ad62c7bc65e9 100644
--- a/third_party/brotli/BUILD.gn
+++ b/third_party/brotli/BUILD.gn
@@ -23,8 +23,17 @@ source_set("brotli") {
     "dec/types.h",
   ]
 
+  config("brotli_warnings") {
+    if (is_clang) {
+      # IncrementalCopyFastPath in decode.c can be unused.
+      # (The file looks very different upstream, this is probably no longer
+      # needed after rolling brotli the next time.)
+      cflags = [ "-Wno-unused-function" ]
+    }
+  }
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
+  configs += [ ":brotli_warnings" ]
 
   # Since we are never debug brotli, freeze the optimizations to -O2.
   if (is_debug) {