Bug 1517071 - Clear AsyncWaitRunnable::mStream when cancelled. r=froydnj

AsyncWaitRunnable holds a strong reference to its stream, and
NonBlockingAsyncInputStream holds a strong reference to the
runnable. The cycle gets broken in the RunAsyncWaitCallback() method
of the stream, but if the runnable is cancelled then we leak them
both. This patch fixes that by clearing the pointer to the stream when
the runnable is cancelled, breaking the cycle.

Differential Revision: https://phabricator.services.mozilla.com/D16248

--HG--
extra : moz-landing-system : lando

testing/web-platform/meta/FileAPI/FileReader/__dir__.ini

@@ -1,2 +0,0 @@
-lsan-allowed: [Alloc, AllocateProtoAndIfaceCache, EntrySlotOrCreate, MakeNotNull, MakeUnique, Malloc, NS_NewCStringInputStream, NewEmptyScopeData, NewPage, ProtoAndIfaceCache, js_new, js_pod_calloc, js_pod_malloc, js_pod_realloc, mozilla::BackgroundHangThread::ReportHang, mozilla::NonBlockingAsyncInputStream::AsyncWait, mozilla::NonBlockingAsyncInputStream::Create, mozilla::dom::Blob::Constructor, mozilla::dom::Blob::Create, mozilla::dom::FileReader::Constructor, mozilla::dom::StringBlobImpl::Create, mozilla::dom::WorkerPrivate::GetOrCreateGlobalScope, mozilla::dom::WorkerPrivate::WorkerPrivate, mozilla::dom::WorkerThread::Create, nsThread::nsThread]
-leak-threshold: [default:51200]

xpcom/io/NonBlockingAsyncInputStream.cpp

@@ -29,6 +29,12 @@ class NonBlockingAsyncInputStream::AsyncWaitRunnable final
     mStream->RunAsyncWaitCallback(this, mCallback.forget());
     return NS_OK;
   }
+
+  NS_IMETHOD
+  Cancel() override {
+    mStream = nullptr;
+    return NS_OK;
+  }
 };
 
 NS_IMPL_ADDREF(NonBlockingAsyncInputStream);