--- a/gfx/layers/ipc/CompositorVsyncScheduler.cpp
+++ b/gfx/layers/ipc/CompositorVsyncScheduler.cpp
@@ -329,10 +329,17 @@ CompositorVsyncScheduler::DispatchVREven
 
 void
 CompositorVsyncScheduler::ScheduleTask(already_AddRefed<CancelableRunnable> aTask)
 {
   MOZ_ASSERT(CompositorThreadHolder::Loop());
   CompositorThreadHolder::Loop()->PostDelayedTask(Move(aTask), 0);
 }
 
+const TimeStamp&
+CompositorVsyncScheduler::GetLastComposeTime() const
+{
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+  return mLastCompose;
+}
+
 } // namespace layers
 } // namespace mozilla

--- a/gfx/layers/ipc/CompositorVsyncScheduler.h
+++ b/gfx/layers/ipc/CompositorVsyncScheduler.h
@@ -78,21 +78,21 @@ public:
   bool NeedsComposite();
 
   /**
    * Force a composite to happen right away, without waiting for the next vsync.
    * This must be called on the compositor thread.
    */
   void ForceComposeToTarget(gfx::DrawTarget* aTarget, const gfx::IntRect* aRect);
 
-  const TimeStamp& GetLastComposeTime()
-  {
-    return mLastCompose;
-  }
-
+  /**
+   * Return the vsync timestamp of the last or ongoing composite. Must be called
+   * on the compositor thread.
+   */
+  const TimeStamp& GetLastComposeTime() const;
 #ifdef COMPOSITOR_PERFORMANCE_WARNING
   const TimeStamp& GetExpectedComposeStartTime()
   {
     return mExpectedComposeStartTime;
   }
 #endif
 
 private: