[Background Fetch] Fix ChromeOS delegate bug.

ChromeOS was creating multiple BackgroundFetchDelegate clients, and
overwriting the client initiating the fetches.

The fix was to check that we are not overwriting the client. This fixes
our problems in the short term, and a longer term fix is planned in
crbug.com/907075.

The fix works on ChromeOS head, as well as 71 (tested via manually
merging the change).

Bug: 907038
Change-Id: Ic1d5bc9f55b22359aa592c71f49d8ab5ed90de5e
Reviewed-on: https://chromium-review.googlesource.com/c/1344030
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609975}

content/browser/background_fetch/background_fetch_delegate_proxy.cc

@@ -92,6 +92,13 @@ class BackgroundFetchDelegateProxy::Core
       std::unique_ptr<BackgroundFetchDescription> fetch_description) {
     DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
+    // If there are multiple clients created we might have registered the wrong
+    // one with the delegate by overwriting it. This check makes sure that we
+    // register the correct client until multiple clients are supported.
+    // TODO(crbug.com/907075): Support multiple clients.
+    if (delegate_ && delegate_->client().get() != this)
+      delegate_->SetDelegateClient(GetWeakPtrOnUI());
+
     if (delegate_)
       delegate_->CreateDownloadJob(std::move(fetch_description));
   }