gistfile1.txt

diff --git a/shell/common/vsync_waiter.cc b/shell/common/vsync_waiter.cc
index b6b55df11..2a21d3fd9 100644
--- a/shell/common/vsync_waiter.cc
+++ b/shell/common/vsync_waiter.cc
@@ -28,9 +28,23 @@ VsyncWaiter::VsyncWaiter(blink::TaskRunners task_runners)
 
 VsyncWaiter::~VsyncWaiter() = default;
 
+// Public method invoked by the animator.
 void VsyncWaiter::AsyncWaitForVsync(Callback callback) {
+  if (!callback) {
+    return;
+  }
+
+  TRACE_EVENT0("flutter", "AsyncWaitForVsync");
+
   {
     std::lock_guard<std::mutex> lock(callback_mutex_);
+    if (callback_) {
+      // The animator may request a frame more than once within a frame
+      // interval. Multiple calls to request frame must result in a single
+      // callback per frame interval.
+      TRACE_EVENT_INSTANT0("flutter", "MultipleCallsToVsyncInFrameInterval");
+      return;
+    }
     callback_ = std::move(callback);
   }
   AwaitVSync();
@@ -46,6 +60,10 @@ void VsyncWaiter::FireCallback(fml::TimePoint frame_start_time,
   }
 
   if (!callback) {
+    // This means that the vsync waiter implementation fired a callback for a
+    // request we did not make. This is a paranoid check but we still want to
+    // make sure we catch misbehaving vsync implementations.
+    TRACE_EVENT_INSTANT0("flutter", "MismatchedFrameCallback");
     return;
   }