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 1094 matching lines @@
         }
 
         void getInvariantOutputCoverage(GrInitInvariantOutput* out) const override {
             out->setUnknownFourComponents();
             out->setUsingLCDCoverage();
         }
 
         const char* name() const override { return "Test LCD Text Batch"; }
         void initBatchTracker(const GrPipelineOptimizations&) override {}
         bool onCombineIfPossible(GrBatch*, const GrCaps&) override  { return false; }
-        void onPrepareDraws(Target*) override {};
+        void onPrepareDraws(Target*) const override {};
 
         typedef GrVertexBatch INHERITED;
     } testLCDCoverageBatch;
 
     GrProcOptInfo colorPOI, covPOI;
     colorPOI.calcColorWithBatch(&testLCDCoverageBatch, nullptr, 0);
     covPOI.calcCoverageWithBatch(&testLCDCoverageBatch, nullptr, 0);
 
     SkASSERT(kRGBA_GrColorComponentFlags == colorPOI.validFlags());
     SkASSERT(covPOI.isFourChannelOutput());
@@ skipping 87 matching lines @@
                 xp->getOptimizations(colorPOI, covPOI, false, 0, caps);
                 TEST_ASSERT(!xp->hasSecondaryOutput());
             }
         }
     }
     ctx->getGpu()->deleteTestingOnlyBackendTexture(backendTex);
 }
 
 #endif