Clean up RPHI IO thread state earlier

This changes RPHI::Cleanup to tear down the corresponding
IOThreadHostImpl, preventing further IO-thread tasks from being handled
by the object. This ensures that no further incoming interface requests
will be handled on the IO thread for the RPHI.

Specifically the purpose of this CL is to avoid any potential interface
binders referencing the host's ResourceContext, which may be destroyed
after Cleanup is run.

Bug: 997634
Change-Id: I25fb96daebd5b2801bb95189e5715986dc4af88c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1773403
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690967}

content/browser/renderer_host/render_process_host_impl.cc

@@ -3443,6 +3443,7 @@ void RenderProcessHostImpl::Cleanup() {
   base::ThreadTaskRunnerHandle::Get()->DeleteSoon(FROM_HERE, this);
   deleting_soon_ = true;
 
+  io_thread_host_impl_.reset();
   if (render_frame_message_filter_) {
     // RenderFrameMessageFilter is refcounted and can outlive the
     // ResourceContext. If the BrowserContext is shutting down, after