Add counting of some GL calls

I would like this facility for tracking FBO switch improvements.

R=bsalomon@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/553583008
diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index e208cdd..eb8455a 100755
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -1941,3 +1941,10 @@
     fResourceCache->printStats();
 }
 #endif
+
+#if GR_GPU_STATS
+const GrContext::GPUStats* GrContext::gpuStats() const {
+    return fGpu->gpuStats();
+}
+#endif
+