Introduce WebPluginContainerBase to abstract WebPluginContainerImpl.

third_party/WebKit/Source/web/WebPluginContainerImpl.cpp

Index: third_party/WebKit/Source/web/WebPluginContainerImpl.cpp
diff --git a/third_party/WebKit/Source/web/WebPluginContainerImpl.cpp b/third_party/WebKit/Source/web/WebPluginContainerImpl.cpp
index 6cbf14726ace79d4fe6c51272924af9033fe9ea4..be8f613bc2f617b7964e92263928cc6760e35540 100644
--- a/third_party/WebKit/Source/web/WebPluginContainerImpl.cpp
+++ b/third_party/WebKit/Source/web/WebPluginContainerImpl.cpp
@@ -665,7 +665,7 @@ bool WebPluginContainerImpl::WantsWheelEvents() {
 
 WebPluginContainerImpl::WebPluginContainerImpl(HTMLPlugInElement* element,
                                                WebPlugin* web_plugin)
-    : ContextClient(element->GetDocument().GetFrame()),
+    : WebPluginContainerBase(element->GetDocument().GetFrame()),
       element_(element),
       web_plugin_(web_plugin),
       web_layer_(nullptr),
@@ -701,8 +701,7 @@ void WebPluginContainerImpl::Dispose() {
 DEFINE_TRACE(WebPluginContainerImpl) {
   visitor->Trace(parent_);
   visitor->Trace(element_);
-  ContextClient::Trace(visitor);
-  PluginView::Trace(visitor);
+  WebPluginContainerBase::Trace(visitor);
 }
 
 void WebPluginContainerImpl::HandleMouseEvent(MouseEvent* event) {