RendererDataMemoizingStore::Store() takes the lock. If the process isn't already being watched, and we are on the UI thread, Store() calls StartObservingProcess(). If StartObservingProcess() does find a RenderProcessHost for the given |process_id|, then it calls RemoveRenderProcessItems(), which tries to take the lock and deadlocks.

I somehow ran into this while writing a ResourceDispatcherHost unit test. I suspect it's not really supposed to happen, because if we are on the UI thread the whole time, a RenderProcessHost that existed when Store() was called can't get destroyed before StartObservingProcess() is called (it's all synchronous).

So maybe Store() should DCHECK that |process_id| corresponds to a valid RPH at the outset.