wire up incNumDraws

BUG=skia:

Review URL: https://codereview.chromium.org/1333623003
diff --git a/src/gpu/GrGpu.cpp b/src/gpu/GrGpu.cpp
index b8cc788..e1b8ed5 100644
--- a/src/gpu/GrGpu.cpp
+++ b/src/gpu/GrGpu.cpp
@@ -366,5 +366,6 @@
     const GrNonInstancedVertices* verts = iter.init(vertices);
     do {
         this->onDraw(args, *verts);
+        fStats.incNumDraws();
     } while ((verts = iter.next()));
 }