cc: Trace the viewport size changing.

This will help in tracking down window resize bugs.

R=enne, vmpstr
BUG=454333

Review URL: https://codereview.chromium.org/864713004

Cr-Commit-Position: refs/heads/master@{#314715}

cc/trees/layer_tree_host_impl.cc

@@ -2195,6 +2195,10 @@ void LayerTreeHostImpl::ReleaseGL() {
 void LayerTreeHostImpl::SetViewportSize(const gfx::Size& device_viewport_size) {
   if (device_viewport_size == device_viewport_size_)
     return;
+  TRACE_EVENT_INSTANT2("cc", "LayerTreeHostImpl::SetViewportSize",
+                       TRACE_EVENT_SCOPE_THREAD, "width",
+                       device_viewport_size.width(), "height",
+                       device_viewport_size.height());
 
   if (pending_tree_)
     active_tree_->SetViewportSizeInvalid();