[Typed OM] Remove MutableStylePropertyMap - it seems unnecessary.

third_party/WebKit/Source/core/css/BUILD.gn

 # 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.
 
 import("//third_party/WebKit/Source/core/core.gni")
 
 blink_core_sources("css") {
   split_count = 5
   sources = [
     "ActiveStyleSheets.cpp",
@@ skipping 279 matching lines @@
     "cssom/CSSUnparsedValue.cpp",
     "cssom/CSSUnparsedValue.h",
     "cssom/CSSUnsupportedStyleValue.cpp",
     "cssom/CSSUnsupportedStyleValue.h",
     "cssom/ComputedStylePropertyMap.cpp",
     "cssom/ComputedStylePropertyMap.h",
     "cssom/FilteredComputedStylePropertyMap.cpp",
     "cssom/FilteredComputedStylePropertyMap.h",
     "cssom/InlineStylePropertyMap.cpp",
     "cssom/InlineStylePropertyMap.h",
-    "cssom/MutableStylePropertyMap.h",
     "cssom/StylePropertyMap.cpp",
     "cssom/StylePropertyMap.h",
     "cssom/StylePropertyMapReadonly.cpp",
     "cssom/StylePropertyMapReadonly.h",
     "cssom/StyleValueFactory.cpp",
     "cssom/StyleValueFactory.h",
     "cssom/WindowGetComputedStyle.h",
     "invalidation/InvalidationSet.cpp",
     "invalidation/InvalidationSet.h",
     "invalidation/PendingInvalidations.h",
@@ skipping 203 matching lines @@
     "resolver/TransformBuilder.h",
     "resolver/ViewportStyleResolver.cpp",
     "resolver/ViewportStyleResolver.h",
   ]
 
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 }