Add operator<< to StyleDifference for debug logging.

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 25 matching lines @@
     "QuotesData.cpp",
     "QuotesData.h",
     "ShadowData.cpp",
     "ShadowData.h",
     "ShadowList.cpp",
     "ShadowList.h",
     "StyleBackgroundData.cpp",
     "StyleBoxData.cpp",
     "StyleContentAlignmentData.h",
     "StyleDeprecatedFlexibleBoxData.cpp",
+    "StyleDifference.cpp",
     "StyleFetchedImage.cpp",
     "StyleFetchedImageSet.cpp",
     "StyleFilterData.cpp",
     "StyleFlexibleBoxData.cpp",
     "StyleGeneratedImage.cpp",
     "StyleGridData.cpp",
     "StyleGridItemData.cpp",
     "StyleImage.cpp",
     "StyleInheritedData.cpp",
     "StyleInheritedVariables.cpp",
@@ skipping 28 matching lines @@
 blink_core_sources("svg") {
   sources = [
     "SVGComputedStyle.cpp",
     "SVGComputedStyleDefs.cpp",
   ]
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 }