[iOS] Define build/config/ios:ios_dynamic_flags for default linker flags.

build/config/ios/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/ios/ios_sdk.gni")
 import("//build/config/sysroot.gni")
 import("//build/toolchain/toolchain.gni")
 
 # This is included by reference in the //build/config/compiler:runtime_library
 # config that is applied to all targets. It is here to separate out the logic
@@ skipping 19 matching lines @@
 
   # TODO(ios): Remove once Xcode's libc++ has LLVM r256325.
   if (use_xcode_clang) {
     cflags += [
       "-isystem",
       rebase_path("//third_party/llvm-build/Release+Asserts/include/c++/v1",
                   root_build_dir),
     ]
   }
 }
+
+config("ios_dynamic_flags") {
+  ldflags = [
+    "-Wl,-ObjC",  # Always load Objective-C categories and class.
+    "-Xlinker",
+    "-objc_abi_version",

[Robert Sesek, 2016/07/20 16:21:07]

I'm not familiar with this. Why is it required?

+    "-Xlinker",
+    "2",
+  ]
+}