Fixed a crash in Background Fetch

Background fetch wasn't unregistering the provider on destruction which
is why there was a crash when trying to open download home.

Bug: 877670
Change-Id: I955a65694fb67aa8d4285a8e18ff2a37f2b220cc
Reviewed-on: https://chromium-review.googlesource.com/1211623Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589370}

chrome/browser/background_fetch/background_fetch_delegate_impl.cc

@@ -46,7 +46,9 @@ BackgroundFetchDelegateImpl::BackgroundFetchDelegateImpl(
   offline_content_aggregator_->RegisterProvider(provider_namespace_, this);
 }
 
-BackgroundFetchDelegateImpl::~BackgroundFetchDelegateImpl() = default;
+BackgroundFetchDelegateImpl::~BackgroundFetchDelegateImpl() {
+  offline_content_aggregator_->UnregisterProvider(provider_namespace_);
+}
 
 download::DownloadService* BackgroundFetchDelegateImpl::GetDownloadService() {
   if (download_service_)