Update existing uses of /wd4267 to use the GN config

extensions/common/BUILD.gn

Index: extensions/common/BUILD.gn
diff --git a/extensions/common/BUILD.gn b/extensions/common/BUILD.gn
index faa1cb88eb619068cb4e6ab791ef72051211b5fe..3cd81177ed1850385c7b260f5fb7315209010f08 100644
--- a/extensions/common/BUILD.gn
+++ b/extensions/common/BUILD.gn
@@ -13,10 +13,8 @@ source_set("common_constants") {
       rebase_path(extensions_gypi_values.extensions_common_constants_sources,
                   ".",
                   "//extensions")
-
-  if (is_win) {
-    cflags = [ "/wd4267" ]  # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
-  }
+  # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
+  configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 }
 
 if (enable_extensions) {
@@ -31,6 +29,8 @@ if (enable_extensions) {
     sources = rebase_path(extensions_gypi_values.extensions_common_sources,
                           ".",
                           "//extensions")
+    # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
+    configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 
     deps = [
       ":common_constants",
@@ -81,9 +81,5 @@ if (enable_extensions) {
                       "//extensions")
       sources += nacl_sources
     }
-
-    if (is_win) {
-      cflags = [ "/wd4267" ]  # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
-    }
   }
 }  # enable_extensions