Pass -Wno-unused-funciton, -Wno-sign-compare in non-clang gn builds.

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/arm.gni")
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//third_party/libvpx/libvpx_srcs.gni")
 import("//third_party/yasm/yasm_assemble.gni")
 
@@ skipping 39 matching lines @@
 config("libvpx_warnings") {
   if (is_clang) {
     cflags = [
       # libvpx heavily relies on implicit enum casting.
       "-Wno-conversion",
       # libvpx does `if ((a == b))` in some places.
       "-Wno-parentheses-equality",
       # libvpx has many static functions in header, which trigger this warning.
       "-Wno-unused-function",
     ]
+  } else if (!is_win) {
+    cflags = [ "-Wno-unused-function", "-Wno-sign-compare" ]
   }
 }
 
 # This config is applied to targets that depend on libvpx.
 config("libvpx_external_config") {
   include_dirs = [
     "//third_party/libvpx/source/libvpx",
   ]
 }
 
@@ skipping 186 matching lines @@
   }
   if (is_android) {
     deps += [ "//third_party/android_tools:cpu_features" ]
   }
   if (current_cpu == "arm") {
     deps += [ ":libvpx_assembly_arm" ]
   }
 
   public_configs = [ ":libvpx_external_config" ]
 }