Add assignment op to enum bitfield ops

src/gpu/effects/GrCustomXfermode.cpp

Index: src/gpu/effects/GrCustomXfermode.cpp
diff --git a/src/gpu/effects/GrCustomXfermode.cpp b/src/gpu/effects/GrCustomXfermode.cpp
index 0a525b4d9279933a4334b1fe19c7ac89a5cc020a..dff2d28781b72b94d74e4fb8bf8f49c59c2d5d29 100644
--- a/src/gpu/effects/GrCustomXfermode.cpp
+++ b/src/gpu/effects/GrCustomXfermode.cpp
@@ -751,10 +751,10 @@ GrXferProcessor::OptFlags CustomXP::onGetOptimizations(const GrProcOptInfo& colo
 
     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.