Remove last uses of InterfaceImpl<> in Sky C++ code

sky/viewer/content_handler_impl.h

Index: sky/viewer/content_handler_impl.h
diff --git a/sky/viewer/content_handler_impl.h b/sky/viewer/content_handler_impl.h
index 8e6f55224c6476d7d991761975dda1104f24776b..d5caf0c75591564a7af07bf0780c50db034f56f4 100644
--- a/sky/viewer/content_handler_impl.h
+++ b/sky/viewer/content_handler_impl.h
@@ -6,15 +6,17 @@
 #define SKY_VIEWER_CONTENT_HANDLER_IMPL_H_
 
 #include "base/message_loop/message_loop.h"
+#include "mojo/public/cpp/bindings/strong_binding.h"
 #include "mojo/public/interfaces/application/shell.mojom.h"
 #include "mojo/services/content_handler/public/interfaces/content_handler.mojom.h"
 
 namespace sky {
 class DocumentView;
 
-class ContentHandlerImpl : public mojo::InterfaceImpl<mojo::ContentHandler> {
+class ContentHandlerImpl : public mojo::ContentHandler {
  public:
-  ContentHandlerImpl();
+  explicit ContentHandlerImpl(
+      mojo::InterfaceRequest<mojo::ContentHandler> request);
   ~ContentHandlerImpl() override;
 
  private:
@@ -22,6 +24,7 @@ class ContentHandlerImpl : public mojo::InterfaceImpl<mojo::ContentHandler> {
   void StartApplication(mojo::InterfaceRequest<mojo::Application> application,
                         mojo::URLResponsePtr response) override;
 
+  mojo::StrongBinding<mojo::ContentHandler> binding_;
   DISALLOW_COPY_AND_ASSIGN(ContentHandlerImpl);
 };