Add unit tests for stroke rects and drawVertices

TBR=bsalomon@google.com
BUG=skia:

Review URL: https://codereview.chromium.org/1133613004
diff --git a/include/gpu/GrTypes.h b/include/gpu/GrTypes.h
index fceae19..67cf4db 100644
--- a/include/gpu/GrTypes.h
+++ b/include/gpu/GrTypes.h
@@ -142,7 +142,8 @@
     kTriangleFan_GrPrimitiveType,
     kPoints_GrPrimitiveType,
     kLines_GrPrimitiveType,     // 1 pix wide only
-    kLineStrip_GrPrimitiveType  // 1 pix wide only
+    kLineStrip_GrPrimitiveType, // 1 pix wide only
+    kLast_GrPrimitiveType = kLineStrip_GrPrimitiveType
 };
 
 static inline bool GrIsPrimTypeLines(GrPrimitiveType type) {