Add GrShaderFlags enum

Replaces GrGLSLUniformHandler::ShaderVisibility with GrShaderFlags.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1684063006

Review URL: https://codereview.chromium.org/1684063006
diff --git a/src/effects/SkDisplacementMapEffect.cpp b/src/effects/SkDisplacementMapEffect.cpp
index e2b7922..ec306cc 100644
--- a/src/effects/SkDisplacementMapEffect.cpp
+++ b/src/effects/SkDisplacementMapEffect.cpp
@@ -539,7 +539,7 @@
     const GrDisplacementMapEffect& displacementMap = args.fFp.cast<GrDisplacementMapEffect>();
     const GrTextureDomain& domain = displacementMap.domain();
 
-    fScaleUni = args.fUniformHandler->addUniform(GrGLSLUniformHandler::kFragment_Visibility,
+    fScaleUni = args.fUniformHandler->addUniform(kFragment_GrShaderFlag,
                                                  kVec2f_GrSLType, kDefault_GrSLPrecision, "Scale");
     const char* scaleUni = args.fUniformHandler->getUniformCStr(fScaleUni);
     const char* dColor = "dColor";