Introduce WebPluginContainerBase to abstract WebPluginContainerImpl.

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

 # Copyright 2017 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("exported") {
   sources = [
     "SharedWorkerRepositoryClientImpl.cpp",
     "SharedWorkerRepositoryClientImpl.h",
     "WebArrayBuffer.cpp",
@@ skipping 11 matching lines @@
     "WebDataSourceImpl.h",
     "WebDateTimeSuggestion.cpp",
     "WebFileChooserCompletionImpl.cpp",
     "WebFileChooserCompletionImpl.h",
     "WebHeap.cpp",
     "WebHistoryItem.cpp",
     "WebImageCache.cpp",
     "WebImageDecoder.cpp",
     "WebMemoryStatistics.cpp",
     "WebPerformance.cpp",
+    "WebPluginContainerBase.cpp",
+    "WebPluginContainerBase.h",
     "WebRange.cpp",
     "WebRenderTheme.cpp",
     "WebScriptController.cpp",
     "WebScriptSource.cpp",
     "WebSecurityPolicy.cpp",
     "WebSelection.cpp",
     "WebSelector.cpp",
     "WebSerializedScriptValue.cpp",
     "WebViewBase.h",
   ]
 
   defines = [ "BLINK_IMPLEMENTATION=1" ]
 }