Fix warning 4373.

Looks like all the current warnings were because of a MSVC bug:
https://github.com/google/googletest/blob/master/googlemock/docs/FrequentlyAskedQuestions.md

We can just disable this one for all tests and be done with it.

Bug: webrtc:261
Change-Id: I882a577f832ff71ac61936abebe0ca537088bab8
Reviewed-on: https://webrtc-review.googlesource.com/40840
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21691}

test/BUILD.gn

diff --git a/test/BUILD.gn b/test/BUILD.gn
index d7a2e77..8eaaae3 100644
--- a/test/BUILD.gn
+++ b/test/BUILD.gn
@@ -149,10 +149,23 @@
   }
 }
 
+config("suppress_warning_4373") {
+  if (is_win) {
+    cflags = [
+      # MSVC has a bug which generates this warning when using mocks; see the
+      # section on warning 4373 in he googlemock FAQ. This warning isn't the
+      # least relevant for us, anyway.
+      "/wd4373",
+    ]
+  }
+}
+
 rtc_source_set("test_support") {
   visibility = [ "*" ]
   testonly = true
 
+  all_dependent_configs = [ ":suppress_warning_4373" ]
+
   sources = [
     "gmock.h",
     "gtest.h",
@@ -344,11 +357,6 @@
     # TODO(jschuh): Bug 1348: fix this warning.
     configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 
-    if (is_win) {
-      # virtual override w/different const/volatile signature.
-      cflags = [ "/wd4373" ]
-    }
-
     if (!build_with_chromium && is_clang) {
       # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
       suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]