Implement SkGLContext swapBuffers with fence syncs

bench/nanobench.h

Index: bench/nanobench.h
diff --git a/bench/nanobench.h b/bench/nanobench.h
index 1dc0b8b90a3ffa5c065b3afc05fdaca9e4c58b64..f556f7dc14ce460622de4da4e4151b5655aebe8a 100644
--- a/bench/nanobench.h
+++ b/bench/nanobench.h
@@ -63,7 +63,7 @@ struct Target {
     /** 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. */