Make ComputedStyleBase use MemberCopy when copying pointer fields.

third_party/WebKit/Source/core/style/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("rendering") {
   visibility = [ "//third_party/WebKit/Source/core/*" ]
 
   sources = [
@@ skipping 30 matching lines @@
     "FilterOperation.h",
     "FilterOperations.cpp",
     "FilterOperations.h",
     "GridArea.h",
     "GridLength.h",
     "GridPosition.h",
     "GridPositionsResolver.cpp",
     "GridPositionsResolver.h",
     "GridTrackSize.h",
     "LineClampValue.h",
+    "MemberCopy.h",
     "NinePieceImage.cpp",
     "NinePieceImage.h",
     "OutlineValue.h",
     "PaintImages.h",
     "QuotesData.cpp",
     "QuotesData.h",
     "ScrollSnapPoints.h",
     "ShadowData.cpp",
     "ShadowData.h",
     "ShadowList.cpp",
@@ skipping 57 matching lines @@
     "SVGComputedStyle.cpp",
     "SVGComputedStyle.h",
     "SVGComputedStyleDefs.cpp",
     "SVGComputedStyleDefs.h",
   ]
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 }