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/glsl/GrGLSLPrimitiveProcessor.cpp b/src/gpu/glsl/GrGLSLPrimitiveProcessor.cpp
index 23bb249..c2c4d4e 100644
--- a/src/gpu/glsl/GrGLSLPrimitiveProcessor.cpp
+++ b/src/gpu/glsl/GrGLSLPrimitiveProcessor.cpp
@@ -40,7 +40,7 @@
                                                  UniformHandle* colorUniform) {
     SkASSERT(colorUniform);
     const char* stagedLocalVarName;
-    *colorUniform = uniformHandler->addUniform(GrGLSLUniformHandler::kFragment_Visibility,
+    *colorUniform = uniformHandler->addUniform(kFragment_GrShaderFlag,
                                                kVec4f_GrSLType,
                                                kDefault_GrSLPrecision,
                                                "Color",