Disable warning for signed/unsigned mismatch in MSVC

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index ef0003273d22d2e6af1a7ca4f925bff40455d049..2dfb94c8a5b80f78acf0cd5ea4f4f671fd35b7e8 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -49,9 +49,17 @@ config("libsrtp_config") {
   ]
 }
 
+config("libsrtp_warnings") {
+  cflags = []
+  if (is_win) {
+    cflags += [ "/wd4018" ]  # Signed/unsigned mismatch in comparison
+  }
+}
+
 static_library("libsrtp") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
+  configs += [ ":libsrtp_warnings" ]
   public_configs = [ ":libsrtp_config" ]
   sources = [
     # includes
@@ -108,6 +116,15 @@ static_library("libsrtp") {
 
 config("srtp_test_config") {
   include_dirs = [ "test" ]
+
+  configs = [ ":libsrtp_warnings" ]
+
+  cflags = []
+  if (is_win && is_clang) {

[kjellander_chromium, 2016/10/12 22:59:48]

Does this work even if it's not in a config? Because in my experience that was
not the case many times when I was battling with Win Clang errors in WebRTC.
I was just hoping you're certain or has tested this.

[mattdr-at-chromium, 2016/10/12 23:01:26]

This is in a config, now, so it should come in after the cflags in
no_chromium_code.

[kjellander_chromium, 2016/10/12 23:56:52]

Oh, I dunno why I didn't see it was in a config already, sorry ;)

+    # TODO(mattdr): Bring in https://github.com/cisco/libsrtp/pull/213
+    # and remove this.
+    cflags += [ "-Wno-format" ]
+  }
 }
 
 template("srtp_test") {