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/gradients/SkGradientShader.cpp b/src/effects/gradients/SkGradientShader.cpp
index 03ebfd6..7a42903 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -917,26 +917,26 @@
                                       const GrGradientEffect& ge) {
 
     if (SkGradientShaderBase::kTwo_GpuColorType == ge.getColorType()) { // 2 Color case
-        fColorStartUni = uniformHandler->addUniform(GrGLSLUniformHandler::kFragment_Visibility,
+        fColorStartUni = uniformHandler->addUniform(kFragment_GrShaderFlag,
                                                     kVec4f_GrSLType, kDefault_GrSLPrecision,
                                                     "GradientStartColor");
-        fColorEndUni = uniformHandler->addUniform(GrGLSLUniformHandler::kFragment_Visibility,
+        fColorEndUni = uniformHandler->addUniform(kFragment_GrShaderFlag,
                                                   kVec4f_GrSLType, kDefault_GrSLPrecision,
                                                   "GradientEndColor");
 
     } else if (SkGradientShaderBase::kThree_GpuColorType == ge.getColorType()) { // 3 Color Case
-        fColorStartUni = uniformHandler->addUniform(GrGLSLUniformHandler::kFragment_Visibility,
+        fColorStartUni = uniformHandler->addUniform(kFragment_GrShaderFlag,
                                                     kVec4f_GrSLType,  kDefault_GrSLPrecision,
                                                     "GradientStartColor");
-        fColorMidUni = uniformHandler->addUniform(GrGLSLUniformHandler::kFragment_Visibility,
+        fColorMidUni = uniformHandler->addUniform(kFragment_GrShaderFlag,
                                                   kVec4f_GrSLType, kDefault_GrSLPrecision,
                                                   "GradientMidColor");
-        fColorEndUni = uniformHandler->addUniform(GrGLSLUniformHandler::kFragment_Visibility,
+        fColorEndUni = uniformHandler->addUniform(kFragment_GrShaderFlag,
                                                   kVec4f_GrSLType, kDefault_GrSLPrecision,
                                                   "GradientEndColor");
 
     } else { // if not a fast case
-        fFSYUni = uniformHandler->addUniform(GrGLSLUniformHandler::kFragment_Visibility,
+        fFSYUni = uniformHandler->addUniform(kFragment_GrShaderFlag,
                                              kFloat_GrSLType, kDefault_GrSLPrecision,
                                              "GradientYCoordFS");
     }