GN: Replace Windows suppressions of warning 4267 with config.

webrtc/system_wrappers/BUILD.gn

Index: webrtc/system_wrappers/BUILD.gn
diff --git a/webrtc/system_wrappers/BUILD.gn b/webrtc/system_wrappers/BUILD.gn
index 435418fcd197c3a8f8aa43cf38059b27616435b6..a1e97d9d475c32ef27bd36f8347ca84e962289e5 100644
--- a/webrtc/system_wrappers/BUILD.gn
+++ b/webrtc/system_wrappers/BUILD.gn
@@ -121,13 +121,13 @@ static_library("system_wrappers") {
     sources += [ "source/atomic32_darwin.cc" ]
   }
 
+  # TODO(jschuh): Bug 1348: fix this warning.
+  configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
+
   if (is_win) {
     libs += [ "winmm.lib" ]
 
-    cflags = [
-      "/wd4267",  # size_t to int truncation.
-      "/wd4334",  # Ignore warning on shift operator promotion.
-    ]
+    cflags = [ "/wd4334" ]  # Ignore warning on shift operator promotion.
   }
 
   deps += [ "../base:rtc_base_approved" ]