Add missing header files to GN files

third_party/WebKit/Source/core/editing/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("editing") {
   split_count = 5
   sources = [
     "CaretDisplayItemClient.cpp",
@@ skipping 10 matching lines @@
     "EditingBoundary.h",
     "EditingStrategy.cpp",
     "EditingStrategy.h",
     "EditingStyle.cpp",
     "EditingStyle.h",
     "EditingStyleUtilities.cpp",
     "EditingStyleUtilities.h",
     "EditingUtilities.cpp",
     "EditingUtilities.h",
     "Editor.cpp",
+    "Editor.h",
     "EditorKeyBindings.cpp",
     "EphemeralRange.cpp",
     "EphemeralRange.h",
     "FrameCaret.cpp",
     "FrameCaret.h",
     "FrameSelection.cpp",
     "FrameSelection.h",
     "GranularityStrategy.cpp",
     "GranularityStrategy.h",
     "InputMethodController.cpp",
@@ skipping 264 matching lines @@
     "//third_party/WebKit/Source:config",
     "//third_party/WebKit/Source:inside_blink",
   ]
 
   deps = [
     "//testing/gmock",
     "//testing/gtest",
     "//third_party/WebKit/Source/core:core",
   ]
 }