Make PlatformViewport and NativeViewportImpl use Shell* instead of ApplicationImpl*.

shell/android/native_viewport_application_loader.cc

Index: shell/android/native_viewport_application_loader.cc
diff --git a/shell/android/native_viewport_application_loader.cc b/shell/android/native_viewport_application_loader.cc
index 35103c1f5cb1cfd5b18958f7c752d1052a9b1c36..646b9df776930ad6b6cecaff8dd781c23b500284 100644
--- a/shell/android/native_viewport_application_loader.cc
+++ b/shell/android/native_viewport_application_loader.cc
@@ -33,8 +33,8 @@ bool NativeViewportApplicationLoader::ConfigureIncomingConnection(
              InterfaceRequest<mojo::NativeViewport> native_viewport_request) {
         if (!gpu_state_)
           gpu_state_ = new gles2::GpuState();
-        new native_viewport::NativeViewportImpl(app_.get(), false, gpu_state_,
-                                                native_viewport_request.Pass());
+        new native_viewport::NativeViewportImpl(
+            app_->shell(), false, gpu_state_, native_viewport_request.Pass());
       });
   service_provider_impl->AddService<mojo::Gpu>(
       [this](const ConnectionContext& connection_context,