Implement SkGLContext swapBuffers with fence syncs

bench/nanobenchAndroid.cpp

Index: bench/nanobenchAndroid.cpp
diff --git a/bench/nanobenchAndroid.cpp b/bench/nanobenchAndroid.cpp
index 3d5cda49295decdabcbcd50410ed4d988cf35f19..50673c974864577739ecf3bb7157471c173bbae0 100644
--- a/bench/nanobenchAndroid.cpp
+++ b/bench/nanobenchAndroid.cpp
@@ -42,7 +42,9 @@ void HWUITarget::fence() {
     this->renderer.proxy->fence();
 }
 
-bool HWUITarget::needsFrameTiming() const {
+bool HWUITarget::needsFrameTiming(int* frameLag) const {
+    extern int FLAGS_gpuFrameLag;
+    *frameLag = FLAGS_gpuFrameLag;
     return true;
 }