Add GrShaderFlags enum

Replaces GrGLSLUniformHandler::ShaderVisibility with GrShaderFlags.

BUG=skia:
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Review URL: https://codereview.chromium.org/1684063006
diff --git a/src/gpu/effects/GrDistanceFieldGeoProc.cpp b/src/gpu/effects/GrDistanceFieldGeoProc.cpp
index 7a63a62..395de32 100644
--- a/src/gpu/effects/GrDistanceFieldGeoProc.cpp
+++ b/src/gpu/effects/GrDistanceFieldGeoProc.cpp
@@ -49,7 +49,7 @@
         // adjust based on gamma
         const char* distanceAdjustUniName = nullptr;
         // width, height, 1/(3*width)
-        fDistanceAdjustUni = uniformHandler->addUniform(GrGLSLUniformHandler::kFragment_Visibility,
+        fDistanceAdjustUni = uniformHandler->addUniform(kFragment_GrShaderFlag,
                                                         kFloat_GrSLType, kDefault_GrSLPrecision,
                                                         "DistanceAdjust", &distanceAdjustUniName);
 #endif
@@ -316,7 +316,7 @@
                              args.fTransformsOut);
 
         const char* textureSizeUniName = nullptr;
-        fTextureSizeUni = uniformHandler->addUniform(GrGLSLUniformHandler::kFragment_Visibility,
+        fTextureSizeUni = uniformHandler->addUniform(kFragment_GrShaderFlag,
                                                      kVec2f_GrSLType, kDefault_GrSLPrecision,
                                                      "TextureSize", &textureSizeUniName);
 
@@ -594,7 +594,7 @@
 
         // adjust width based on gamma
         const char* distanceAdjustUniName = nullptr;
-        fDistanceAdjustUni = uniformHandler->addUniform(GrGLSLUniformHandler::kFragment_Visibility,
+        fDistanceAdjustUni = uniformHandler->addUniform(kFragment_GrShaderFlag,
                                                         kVec3f_GrSLType, kDefault_GrSLPrecision,
                                                         "DistanceAdjust", &distanceAdjustUniName);
         fragBuilder->codeAppendf("distance -= %s;", distanceAdjustUniName);