exo: Do not capture cursor before initial commit

Bug: 707170
Test: No DCHECK when mouse first enters gtk3-demo.
Change-Id: Id8c7cd5834b8071384be2967c686988e16eadf9e
Reviewed-on: https://chromium-review.googlesource.com/899912Reviewed-by: David Reveman <reveman@chromium.org>
Commit-Queue: Dominik Laskowski <domlaskowski@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534747}

components/exo/pointer.cc

@@ -398,6 +398,10 @@ void Pointer::CaptureCursor(const gfx::Point& hotspot) {
   DCHECK(root_surface());
   DCHECK(focus_surface_);
 
+  // Defer capture until surface commit.
+  if (host_window()->bounds().IsEmpty())
+    return;
+
   // Submit compositor frame to be captured.
   SubmitCompositorFrame();