[Mac/GN] Add missing -Wobjc-missing-property-synthesis.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index afedebc8591906544b5cabfea6bcdab0ed15925c..2554042ef4156170fea2480ecf60922a12b7742e 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1061,6 +1061,13 @@ config("chromium_code") {
       # disabled, so only do that for Release build.
       defines += [ "_FORTIFY_SOURCE=2" ]
     }
+
+    if (is_mac) {
+      # Warn if automatic synthesis is triggered. This triggers numerous
+      # warnings for internal iOS directories.
+      cflags_objc = [ "-Wobjc-missing-property-synthesis" ]
+      cflags_objcc = [ "-Wobjc-missing-property-synthesis" ]
+    }
   }
 
   configs = [ ":default_warnings" ]