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/GrConvexPolyEffect.cpp b/src/gpu/effects/GrConvexPolyEffect.cpp
index fdc4a39..31ace60 100644
--- a/src/gpu/effects/GrConvexPolyEffect.cpp
+++ b/src/gpu/effects/GrConvexPolyEffect.cpp
@@ -104,7 +104,7 @@
     const char *rectName;
     // The rect uniform's xyzw refer to (left + 0.5, top + 0.5, right - 0.5, bottom - 0.5),
     // respectively.
-    fRectUniform = args.fUniformHandler->addUniform(GrGLSLUniformHandler::kFragment_Visibility,
+    fRectUniform = args.fUniformHandler->addUniform(kFragment_GrShaderFlag,
                                                     kVec4f_GrSLType,
                                                     kDefault_GrSLPrecision,
                                                     "rect",
@@ -188,7 +188,7 @@
     const GrConvexPolyEffect& cpe = args.fFp.cast<GrConvexPolyEffect>();
 
     const char *edgeArrayName;
-    fEdgeUniform = args.fUniformHandler->addUniformArray(GrGLSLUniformHandler::kFragment_Visibility,
+    fEdgeUniform = args.fUniformHandler->addUniformArray(kFragment_GrShaderFlag,
                                                          kVec3f_GrSLType,
                                                          kDefault_GrSLPrecision,
                                                          "edges",