Add config to enable ARC on iOS/OS X to //build/config.

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 3b125c21c2472934dc470e0738aa268dfe7ada57..76a42eb399d573f7fce1ad3c5897554af7ce640b 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1335,3 +1335,13 @@ config("default_symbols") {
     assert(false)
   }
 }
+
+if (is_ios || is_mac) {
+  # On Mac and iOS, this enables support for ARC (automatic ref-counting).
+  # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
+  config("enable_arc") {
+    common_flags = [ "-fobjc-arc" ]
+    cflags_objc = common_flags
+    cflags_objcc = common_flags
+  }
+}