Bug 1351263 - Label runnables in dom/media/Benchmark.cpp; r=jwwang

MozReview-Commit-ID: LZygjhx2A7i

--HG--
extra : rebase_source : 738e11624ba9144f87f2db2bc7260480095877d7

dom/media/Benchmark.cpp

@@ -60,8 +60,7 @@ VP9Benchmark::IsVP9DecodeFast()
                         Preferences::GetUint("media.benchmark.timeout", 1000))
                     });
     estimiser->Run()->Then(
-      // Non-DocGroup version of AbstractThread::MainThread for utility function.
-      AbstractThread::MainThread(), __func__,
+      SystemGroup::AbstractMainThreadFor(TaskCategory::Other), __func__,
       [](uint32_t aDecodeFps) {
         if (XRE_IsContentProcess()) {
           dom::ContentChild* contentChild = dom::ContentChild::GetSingleton();