Fixing script injection in app_shell.

Script injection wasn't working in app_shell because the ScriptInjectionManager
never got created.

BUG=352293

Review URL: https://codereview.chromium.org/602563003

Cr-Commit-Position: refs/heads/master@{#296679}

extensions/shell/renderer/shell_content_renderer_client.cc

@@ -111,6 +111,7 @@ void ShellContentRendererClient::RenderFrameCreated(
 void ShellContentRendererClient::RenderViewCreated(
     content::RenderView* render_view) {
   new ExtensionHelper(render_view, extension_dispatcher_.get());
+  extension_dispatcher_->OnRenderViewCreated(render_view);
 }
 
 bool ShellContentRendererClient::OverrideCreatePlugin(