Make onPrepareDraws const

tests/GrPorterDuffTest.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkXfermode.h"
 #include "Test.h"
 
@@ skipping 1100 matching lines @@
                                           GrBatchToXPOverrides* overrides) const override {
             color->setKnownFourComponents(GrColorPackRGBA(123, 45, 67, 221));
             coverage->setUnknownFourComponents();
             coverage->setUsingLCDCoverage();
             overrides->fUsePLSDstRead = false;
         }
 
         const char* name() const override { return "Test LCD Text Batch"; }
         void initBatchTracker(const GrXPOverridesForBatch&) override {}
         bool onCombineIfPossible(GrBatch*, const GrCaps&) override  { return false; }
-        void onPrepareDraws(Target*) override {};
+        void onPrepareDraws(Target*) const override {};
 
         typedef GrVertexBatch INHERITED;
     } testLCDCoverageBatch;
 
     GrPipelineOptimizations opts;
     testLCDCoverageBatch.getPipelineOptimizations(&opts);
     GrProcOptInfo colorPOI = opts.fColorPOI;
     GrProcOptInfo covPOI = opts.fCoveragePOI;
 
     SkASSERT(kRGBA_GrColorComponentFlags == colorPOI.validFlags());
@@ skipping 89 matching lines @@
                 xp->getOptimizations(optimizations, false, 0, caps);
                 TEST_ASSERT(!xp->hasSecondaryOutput());
             }
         }
     }
     ctx->getGpu()->deleteTestingOnlyBackendTexture(backendTex);
 }
 
 #endif