Make onPrepareDraws const

src/gpu/batches/GrTInstanceBatch.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrTInstanceBatch_DEFINED
 #define GrTInstanceBatch_DEFINED
 
@@ skipping 71 matching lines @@
     }
 
     void updateBoundsAfterAppend() {
         const Geometry& geo = fGeoData.back();
         Impl::UpdateBoundsAfterAppend(geo, &fBounds);
     }
 
 private:
     GrTInstanceBatch() : INHERITED(ClassID()) {}
 
-    void onPrepareDraws(Target* target) override {
+    void onPrepareDraws(Target* target) const override {
         SkAutoTUnref<const GrGeometryProcessor> gp(Impl::CreateGP(this->seedGeometry(), 
                                                                   fOverrides));
         if (!gp) {
             SkDebugf("Couldn't create GrGeometryProcessor\n");
             return;
         }
 
         target->initDraw(gp, this->pipeline());
 
         size_t vertexStride = gp->getVertexStride();
@@ skipping 42 matching lines @@
         return true;
     }
 
     GrXPOverridesForBatch fOverrides;
     SkSTArray<1, Geometry, true> fGeoData;
 
     typedef GrVertexBatch INHERITED;
 };
 
 #endif