Make onPrepareDraws const

src/gpu/batches/GrDrawVerticesBatch.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 GrDrawVerticesBatch_DEFINED
 #define GrDrawVerticesBatch_DEFINED
 
@@ skipping 31 matching lines @@
 
     const char* name() const override { return "DrawVerticesBatch"; }
 
     void computePipelineOptimizations(GrInitInvariantOutput* color, 
                                       GrInitInvariantOutput* coverage,
                                       GrBatchToXPOverrides* overrides) const override;
 
     SkSTArray<1, Geometry, true>* geoData() { return &fGeoData; }
 
 private:
-    void onPrepareDraws(Target*) override;
+    void onPrepareDraws(Target*) const override;
     void initBatchTracker(const GrXPOverridesForBatch&) override;
 
     GrDrawVerticesBatch(const Geometry& geometry, GrPrimitiveType primitiveType,
                         const SkMatrix& viewMatrix,
                         const SkPoint* positions, int vertexCount,
                         const uint16_t* indices, int indexCount,
                         const GrColor* colors, const SkPoint* localCoords, const SkRect& bounds);
 
     GrPrimitiveType primitiveType() const { return fPrimitiveType; }
     bool batchablePrimitiveType() const {
@@ skipping 10 matching lines @@
     int                 fVertexCount;
     int                 fIndexCount;
     bool                fCoverageIgnored; // comes from initBatchTracker.
 
     SkSTArray<1, Geometry, true> fGeoData;
 
     typedef GrVertexBatch INHERITED;
 };
 
 #endif