Ensure the frame tree node id is always set after setting the mojo channel.

The input router is recreated after a new input channel is constructed.
Ensure that the Frame Tree Node Id is set in all codepaths.

BUG=827267

Change-Id: Ie8c6a2665afccad23fdea9cfa0dc186c3b919455
Reviewed-on: https://chromium-review.googlesource.com/992357Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547884}

content/browser/frame_host/render_frame_host_impl.cc

@@ -1298,6 +1298,8 @@ void RenderFrameHostImpl::SetRenderFrameCreated(bool created) {
       render_widget_host_->SetWidgetInputHandler(std::move(widget_handler),
                                                  std::move(host_request));
     }
+    render_widget_host_->input_router()->SetFrameTreeNodeId(
+        frame_tree_node_->frame_tree_node_id());
     viz::mojom::InputTargetClientPtr input_target_client;
     remote_interfaces_->GetInterface(&input_target_client);
     input_target_client_ = input_target_client.get();

content/browser/renderer_host/input/input_router.h

@@ -55,7 +55,10 @@ class InputRouter : public IPC::Listener {
   virtual void SetDeviceScaleFactor(float device_scale_factor) = 0;
 
   // Sets the frame tree node id of associated frame, used when tracing
-  // input event latencies to relate events to their target frames.
+  // input event latencies to relate events to their target frames. Since
+  // input always flows to Local Frame Roots, the |frameTreeNodeId| is
+  // relative to the Frame associated with the Local Frame Root for the
+  // widget owning this InputRouter.
   virtual void SetFrameTreeNodeId(int frameTreeNodeId) = 0;
 
   // Return the currently allowed touch-action.