Cherry-pick: Do not request vsync on touch if using synchronous compositor

Clean cherry-pick of chromium crrev.com/r289997

BUG: 17033041

Original description:

Nothing registers for the callback in WasShown if using
synchronous compositor, so this does not change any behavior.
So this is only removing unneeded RequestVSyncUpdate

Change-Id: I7346590b81f03adc31bbc8b8d067d5fe4590582b
diff --git a/content/browser/renderer_host/render_widget_host_view_android.cc b/content/browser/renderer_host/render_widget_host_view_android.cc
index ff24085..4196497 100644
--- a/content/browser/renderer_host/render_widget_host_view_android.cc
+++ b/content/browser/renderer_host/render_widget_host_view_android.cc
@@ -1190,7 +1190,7 @@
   // This is good enough as long as the first touch event has Begin semantics
   // and the actual scroll happens on the next vsync.
   // TODO: Is this actually still needed?
-  if (content_view_core_) {
+  if (content_view_core_ && observing_root_window_) {
     content_view_core_->GetWindowAndroid()->RequestVSyncUpdate();
   }
 }