Bug 1143575. Don't report negative frame delays. r=cpearce

dom/html/HTMLVideoElement.cpp

@@ -174,7 +174,11 @@ double HTMLVideoElement::MozFrameDelay()
 {
   MOZ_ASSERT(NS_IsMainThread(), "Should be on main thread.");
   VideoFrameContainer* container = GetVideoFrameContainer();
-  return container ?  container->GetFrameDelay() : 0;
+  // Hide negative delays. Frame timing tweaks in the compositor (e.g.
+  // adding a bias value to prevent multiple dropped/duped frames when
+  // frame times are aligned with composition times) may produce apparent
+  // negative delay, but we shouldn't report that.
+  return container ? std::max(0.0, container->GetFrameDelay()) : 0.0;
 }
 
 bool HTMLVideoElement::MozHasAudio() const