Code: void VRDisplay::ProcessScheduledWindowAnimations(double timestamp) {
  TRACE_EVENT1("gpu", "VRDisplay::window.rAF", "frame", vr_frame_id_);
  auto doc = navigator_vr_->GetDocument();
  if (!doc)
    return;
   auto page = doc->GetPage();
   if (!page)
     return;
   page->Animator().ServiceScriptedAnimations(timestamp);
 }

Vulnerability Type: DoS 
CWE ID: 
Summary: Multiple unspecified vulnerabilities in Google Chrome before 43.0.2357.65 allow attackers to cause a denial of service or possibly have other impact via unknown vectors.
Commit Message: WebVR: fix initial vsync

Applications sometimes use window.rAF while not presenting, then switch to
vrDisplay.rAF after presentation starts. Depending on the animation loop's
timing, this can cause a race condition where presentation has been started
but there's no vrDisplay.rAF pending yet. Ensure there's at least vsync
being processed after presentation starts so that a queued window.rAF
can run and schedule a vrDisplay.rAF.

BUG=711789

Review-Url: https://codereview.chromium.org/2848483003
Cr-Commit-Position: refs/heads/master@{#468167}