Implement SkGLContext swapBuffers with fence syncs

bench/nanobench.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef nanobench_DEFINED
 #define nanobench_DEFINED
 
@@ skipping 45 matching lines @@
     virtual void endTiming() { }
 
     /** Called between benchmarks (or between calibration and measured
         runs) to make sure all pending work in drivers / threads is
         complete. */
     virtual void fence() { }
 
     /** CPU-like targets can just be timed, but GPU-like
         targets need to pay attention to frame boundaries
         or other similar details. */
-    virtual bool needsFrameTiming() const { return false; }
+    virtual bool needsFrameTiming(int* frameLag) const { return false; }
 
     /** Called once per target, during program initialization.
         Returns false if initialization fails. */
     virtual bool init(SkImageInfo info, Benchmark* bench);
 
     /** Stores any pixels drawn to the screen in the bitmap.
         Returns false on error. */
     virtual bool capturePixels(SkBitmap* bmp);
 
     /** Writes any config-specific data to the log. */
     virtual void fillOptions(ResultsWriter*) { }
 
     SkCanvas* getCanvas() const {
         if (!surface.get()) {
             return NULL;
         }
         return surface->getCanvas();
     }
 };
 
 #endif  // nanobench_DEFINED