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

third_party/brotli/brotli.gyp

Index: third_party/brotli/brotli.gyp
diff --git a/third_party/brotli/brotli.gyp b/third_party/brotli/brotli.gyp
index 9c47b1fa607e08e20045370189a1fb85972bcbc0..38c472976dc5c87d327a826acf5f2f00c2ca5ee3 100644
--- a/third_party/brotli/brotli.gyp
+++ b/third_party/brotli/brotli.gyp
@@ -29,6 +29,14 @@
         'dec/transform.h',
         'dec/types.h',
       ],
+      'variables': {
+        'clang_warning_flags': [
+          # 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.)
+          '-Wno-unused-function',
+        ],
+      },
       'conditions': [
         ['os_posix==1 and (target_arch=="arm" or target_arch=="armv7" or target_arch=="arm64")', {
           'cflags!': ['-Os'],