media: Use task runner for testing in MockPeerConnectionTrackerHost

This is part of efforts to replace base::ThreadTaskRunnerHandle::Get()
and SequencedTaskRunnerHandle::Get() with other appropriate task runners
in the renderer.

Bug: 780785, 786332
Change-Id: Ic9801a4896dd4d9a70f9b9fa4f5b26dc474db224
Reviewed-on: https://chromium-review.googlesource.com/930524Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539071}

content/renderer/media/webrtc/peer_connection_tracker_unittest.cc

@@ -40,7 +40,8 @@ class MockPeerConnectionTrackerHost : public mojom::PeerConnectionTrackerHost {
     mojom::PeerConnectionTrackerHostAssociatedPtr
         peer_connection_tracker_host_ptr_;
     binding_.Bind(mojo::MakeRequestAssociatedWithDedicatedPipe(
-        &peer_connection_tracker_host_ptr_));
+                      &peer_connection_tracker_host_ptr_),
+                  blink::scheduler::GetSingleThreadTaskRunnerForTesting());
     return peer_connection_tracker_host_ptr_;
   }
   mojo::AssociatedBinding<mojom::PeerConnectionTrackerHost> binding_;