fix for GrPorterDuffTest valgrind error

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 1175 matching lines @@
         kNone_GrColorComponentFlags,
         kG_GrColorComponentFlag | kA_GrColorComponentFlag,
         kRGBA_GrColorComponentFlags
     };
     GR_STATIC_ASSERT(SK_ARRAY_COUNT(testColors) == SK_ARRAY_COUNT(testColorFlags));
 
     for (size_t c = 0; c < SK_ARRAY_COUNT(testColors); c++) {
         GrPipelineOptimizations optimizations;
         optimizations.fColorPOI.calcWithInitialValues(nullptr, 0, testColors[c], testColorFlags[c], 
                                                       false);
+        optimizations.fOverrides.fUsePLSDstRead = false;
         for (int f = 0; f <= 1; f++) {
             if (!f) {
                 optimizations.fCoveragePOI.calcWithInitialValues(nullptr, 0, 0, 
                                                                  kNone_GrColorComponentFlags, true);
             } else {
                 optimizations.fCoveragePOI.calcWithInitialValues(nullptr, 0, GrColorPackA4(255),
                                                                  kRGBA_GrColorComponentFlags, true);
             }
             for (int m = 0; m <= SkXfermode::kLastCoeffMode; m++) {
                 SkXfermode::Mode xfermode = static_cast<SkXfermode::Mode>(m);
@@ skipping 10 matching lines @@
                 xp->getOptimizations(optimizations, false, 0, caps);
                 TEST_ASSERT(!xp->hasSecondaryOutput());
             }
         }
     }
     ctx->getGpu()->deleteTestingOnlyBackendTexture(backendTex);
 }
 
 #endif