Push service may be null and must only be used on the UI thread.

TBR=avi@chromium.org for comment-only change in content/public

Review URL: https://codereview.chromium.org/780783004

Cr-Commit-Position: refs/heads/master@{#307037}

content/browser/push_messaging/push_messaging_message_filter.cc

@@ -206,6 +206,11 @@ void PushMessagingMessageFilter::DoPermissionStatusRequest(
     int render_frame_id,
     int callback_id) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  if (!service()) {
+    Send(new PushMessagingMsg_PermissionStatusFailure(render_frame_id,
+                                                      callback_id));
+    return;
+  }
   blink::WebPushPermissionStatus permission_value =
       service()->GetPermissionStatus(
           requesting_origin, render_process_id_, render_frame_id);
@@ -218,6 +223,10 @@ void PushMessagingMessageFilter::GetPermissionStatusOnUI(
     const GURL& requesting_origin,
     int request_id) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  if (!service()) {
+    Send(new PushMessagingMsg_GetPermissionStatusError(request_id));
+    return;
+  }
   GURL embedding_origin = requesting_origin;
   blink::WebPushPermissionStatus permission_status =
       service()->GetPermissionStatus(requesting_origin, embedding_origin);
@@ -264,6 +273,7 @@ void PushMessagingMessageFilter::SendRegisterSuccess(
 }
 
 PushMessagingService* PushMessagingMessageFilter::service() {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
   if (!service_) {
     RenderProcessHost* process_host =
         RenderProcessHost::FromID(render_process_id_);

content/browser/push_messaging/push_messaging_message_filter.h

@@ -82,6 +82,9 @@ class PushMessagingMessageFilter : public BrowserMessageFilter {
   void SendRegisterSuccess(const RegisterData& data,
                            const std::string& push_registration_id);
 
+  // Returns a push messaging service. The embedder owns the service, and is
+  // responsible for ensuring that it outlives RenderProcessHost. It's valid to
+  // return NULL. Must be called on the UI thread.
   PushMessagingService* service();
 
   int render_process_id_;

content/public/browser/push_messaging_service.h

@@ -16,7 +16,7 @@
 namespace content {
 
 // A push service-agnostic interface that the Push API uses for talking to
-// push messaging services like GCM.
+// push messaging services like GCM. Must only be used on the UI thread.
 class CONTENT_EXPORT PushMessagingService {
  public:
   typedef base::Callback<void(const std::string& /* registration_id */,