Split GrDrawBatch and GrVertexBatch into their own files

TBR=joshualitt@google.com

Review URL: https://codereview.chromium.org/1296483002
diff --git a/gm/convexpolyeffect.cpp b/gm/convexpolyeffect.cpp
index 1c4a0b1..e9b417f 100644
--- a/gm/convexpolyeffect.cpp
+++ b/gm/convexpolyeffect.cpp
@@ -23,6 +23,7 @@
 #include "SkTLList.h"
 
 #include "batches/GrTestBatch.h"
+#include "batches/GrVertexBatch.h"
 
 #include "effects/GrConvexPolyEffect.h"