Add outline files for StylePropertyMap.

third_party/WebKit/Source/core/css/cssom/MutableStylePropertyMap.h

Index: third_party/WebKit/Source/core/css/cssom/MutableStylePropertyMap.h
diff --git a/third_party/WebKit/Source/core/css/cssom/MutableStylePropertyMap.h b/third_party/WebKit/Source/core/css/cssom/MutableStylePropertyMap.h
new file mode 100644
index 0000000000000000000000000000000000000000..be9f2d452fa9f1655566f6d700b33aa15dbcdf6c
--- /dev/null
+++ b/third_party/WebKit/Source/core/css/cssom/MutableStylePropertyMap.h
@@ -0,0 +1,18 @@
+// 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.
+
+#ifndef MutableStylePropertyMap_h
+#define MutableStylePropertyMap_h
+
+#include "core/css/cssom/StylePropertyMap.h"
+
+namespace blink {
+
+class CORE_EXPORT MutableStylePropertyMap : public StylePropertyMap {
+    WTF_MAKE_NONCOPYABLE(MutableStylePropertyMap);
+};
+
+} // namespace blink
+
+#endif