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

third_party/snappy/BUILD.gn

Index: third_party/snappy/BUILD.gn
diff --git a/third_party/snappy/BUILD.gn b/third_party/snappy/BUILD.gn
index e8de575e40ea0a6f40c55be53fb58d2fcc71a79b..97cce89bc719d8425d88b54a724bb0d758027c81 100644
--- a/third_party/snappy/BUILD.gn
+++ b/third_party/snappy/BUILD.gn
@@ -27,8 +27,16 @@ static_library("snappy") {
     "src/snappy.h",
   ]
 
+  config("snappy_warnings") {
+    if (is_clang) {
+      # ComputeTable is unused,
+      # https://code.google.com/p/snappy/issues/detail?id=96
+      cflags = [ "-Wno-unused-function" ]
+    }
+  }
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
+  configs += [ ":snappy_warnings" ]
   public_configs = [ ":snappy_config" ]
 
   if (is_win) {