Make onPrepareDraws const

src/gpu/batches/GrVertexBatch.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 GrVertexBatch_DEFINED
 #define GrVertexBatch_DEFINED
 
@@ skipping 47 matching lines @@
 
         using InstancedHelper::recordDraw;
     private:
         typedef InstancedHelper INHERITED;
     };
 
 private:
     void onPrepare(GrBatchFlushState* state) final;
     void onDraw(GrBatchFlushState* state) final;
 
-    virtual void onPrepareDraws(Target*) = 0;
+    virtual void onPrepareDraws(Target*) const = 0;
 
     // A set of contiguous draws with no inline uploads between them that all use the same
     // primitive processor. All the draws in a DrawArray share a primitive processor and use the
     // the batch's GrPipeline.
     struct DrawArray {
         SkSTArray<1, GrVertices, true>                      fDraws;
         GrPendingProgramElement<const GrPrimitiveProcessor> fPrimitiveProcessor;
     };
 
     // Array of DrawArray. There may be inline uploads between each DrawArray and each DrawArray
     // may use a different primitive processor.
     typedef SkTLList<DrawArray, 4> DrawArrayList;
     DrawArrayList fDrawArrays;
 
     typedef GrDrawBatch INHERITED;
 };
 
 #endif