Implement SkGLContext swapBuffers with fence syncs

Improves the GPU measuring accuracy of nanobench by using fence syncs.
Fence syncs are very widely supported and available on almost every
platform.

NO_MERGE_BUILDS
BUG=skia:

Review URL: https://codereview.chromium.org/1194783003
diff --git a/include/gpu/gl/GrGLInterface.h b/include/gpu/gl/GrGLInterface.h
index e510551..8c72526 100644
--- a/include/gpu/gl/GrGLInterface.h
+++ b/include/gpu/gl/GrGLInterface.h
@@ -30,6 +30,8 @@
  * comments in GrGLConfig.h
  */
 
+typedef void(*GrGLFuncPtr)();
+
 struct GrGLInterface;
 
 const GrGLInterface* GrGLDefaultInterface();