Some more improvements/fixes to the VulkanViewer

tools/vulkan/viewer/VulkanViewer.h

 /*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */
 
 #ifndef VulkanViewer_DEFINED
 #define VulkanViewer_DEFINED
 
 #include "../Application.h"
 #include "../Window.h"
 #include "gm.h"
 
 class SkCanvas;
 
 class VulkanViewer : public Application {
 public:
     VulkanViewer(int argc, char** argv, void* platformData);
     ~VulkanViewer() override;
 
     bool onKey(Window::Key key, Window::InputState state, uint32_t modifiers);
     void onPaint(SkCanvas* canvas);
 
-    void onIdle(float dt) override;
+    void onIdle(double ms) override;
 
 private:
+    void drawStats(SkCanvas* canvas);
+
     Window*      fWindow;
 
+    static const int kMeasurementCount = 64;  // should be power of 2 for fast mod
+    double fMeasurements[kMeasurementCount];
+    int fCurrentMeasurement;
+
     const skiagm::GMRegistry* fGMs;
 };
 
 
 #endif