Added AddDownload communication path and observers

content/browser/background_fetch/background_fetch_context.cc

Index: content/browser/background_fetch/background_fetch_context.cc
diff --git a/content/browser/background_fetch/background_fetch_context.cc b/content/browser/background_fetch/background_fetch_context.cc
index 2e087b26459a85969ffd42e8672c0b682521e638..c69dab32f4816ca2603f8e2239fc6c25945cc980 100644
--- a/content/browser/background_fetch/background_fetch_context.cc
+++ b/content/browser/background_fetch/background_fetch_context.cc
@@ -45,7 +45,8 @@ BackgroundFetchContext::BackgroundFetchContext(
           base::MakeUnique<BackgroundFetchDataManager>(browser_context)),
       event_dispatcher_(base::MakeUnique<BackgroundFetchEventDispatcher>(
           std::move(service_worker_context))),
-      client_proxy_(base::MakeUnique<BackgroundFetchClientProxy>(this)) {
+      client_proxy_(
+          base::MakeUnique<BackgroundFetchClientProxy>(browser_context, this)) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   BackgroundFetchClient* background_fetch_client =
       browser_context_->GetBackgroundFetchClient();
@@ -103,6 +104,13 @@ void BackgroundFetchContext::DidCreateRegistration(
   // Create the BackgroundFetchJobController, which will do the actual fetching.
   CreateController(registration_id, options, std::move(initial_requests));
 
+  // Tell the BackgroundFetchClient that a job has started, which will allow
+  // embedders to display updates.
+  BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
+                          base::Bind(&BackgroundFetchClientProxy::AddDownload,
+                                     client_proxy_->GetWeakPtr(),
+                                     registration_id.Serialize(), options));

[Peter Beverloo, 2017/04/20 16:27:55]

I'd like to encapsulate the thread-jumps to the BackgroundFetchClientProxy.
Based on the contract defined in the class-level comment it's fine to call these
methods on the IO thread.

In order to make this fully safe, we should:

    1) Initialise the BackgroundFetchClientProxy on the UI thread, have it store
the BackgroundFetchClient pointer. (Retrieved from the BrowserContext.)
    2) In BackgroundFetchContext::Shutdown, call through to the BGFCP where we
set the client_ to a nullptr.
    3) For calls like this,
        a) Call BGFCP::AddFrobinator() on the IO thread. Jump to
BGFCP::AddFrobinatorOnUI w/ a WeakPtr.
        b) Check if client_ is not a nullptr. Make the call if so.

Downside is needing two methods per call, but it avoids making the assumption
that the browser_context will remain alive, while also avoiding lots of null
checks here or in the JobController.

[Peter Beverloo, 2017/04/20 16:27:55]

Should the JobController make this call? It has better knowledge of the state of
an in-progress fetch than the context.

[Peter Beverloo, 2017/04/24 14:39:14]

Ping.

+
   // Create the BackgroundFetchRegistration the renderer process will receive,
   // which enables it to resolve the promise telling the developer it worked.
   BackgroundFetchRegistration registration;