Implement ComputedStylePropertyMap for Typed OM.

third_party/WebKit/Source/core/css/cssom/WindowGetComputedStyle.idl

Index: third_party/WebKit/Source/core/css/cssom/WindowGetComputedStyle.idl
diff --git a/third_party/WebKit/Source/core/css/cssom/WindowGetComputedStyle.idl b/third_party/WebKit/Source/core/css/cssom/WindowGetComputedStyle.idl
new file mode 100644
index 0000000000000000000000000000000000000000..e70c68d077c08c26fd1af320224ba6c32dda7912
--- /dev/null
+++ b/third_party/WebKit/Source/core/css/cssom/WindowGetComputedStyle.idl
@@ -0,0 +1,11 @@
+// Copyright 2016 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.
+
+// https://drafts.css-houdini.org/css-typed-om/#computed-stylepropertymap-objects
+
+[
+    RuntimeEnabled=CSSTypedOM,
+] partial interface Window {
+    [NewObject] StylePropertyMap getComputedStyleMap(Element element, optional DOMString? pseudoElement = null);
+};