Add assignment op to enum bitfield ops

src/gpu/effects/GrCustomXfermode.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 "effects/GrCustomXfermode.h"
 #include "effects/GrCustomXfermodePriv.h"
 
@@ skipping 733 matching lines @@
       f * blend(Sa, Da) + (1-f) * Da
         = f * (Sa + Da - Sa * Da) + (1-f) * Da
         = f*Sa + f*Da - f*Sa * Da + Da - f*Da
         = f*Sa - f*Sa * Da + Da
         = f*Sa + Da - f*Sa * Da
         = blend(f*Sa, Da)
    */
 
     OptFlags flags = kNone_Opt;
     if (colorPOI.allStagesMultiplyInput()) {
-        flags = flags | kCanTweakAlphaForCoverage_OptFlag;
+        flags |= kCanTweakAlphaForCoverage_OptFlag;
     }
     if (coveragePOI.isSolidWhite()) {
-        flags = flags | kIgnoreCoverage_OptFlag;
+        flags |= kIgnoreCoverage_OptFlag;
     }
     if (caps.advancedBlendEquationSupport() && !coveragePOI.isFourChannelOutput()) {
         // This blend mode can be implemented in hardware.
         fHWBlendEquation = hw_blend_equation(fMode);
     }
     return flags;
 }
 
 bool CustomXP::onWillNeedXferBarrier(const GrRenderTarget* rt,
                                      const GrDrawTargetCaps& caps,
@@ skipping 50 matching lines @@
 GR_DEFINE_XP_FACTORY_TEST(GrCustomXPFactory);
 GrXPFactory* GrCustomXPFactory::TestCreate(SkRandom* rand,
                                            GrContext*,
                                            const GrDrawTargetCaps&,
                                            GrTexture*[]) {
     int mode = rand->nextRangeU(SkXfermode::kLastCoeffMode + 1, SkXfermode::kLastSeparableMode);
 
     return SkNEW_ARGS(GrCustomXPFactory, (static_cast<SkXfermode::Mode>(mode)));
 }