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/GrGLSLShaderBuilder.cpp b/src/gpu/glsl/GrGLSLShaderBuilder.cpp
index a2cf66c..c94a855 100644
--- a/src/gpu/glsl/GrGLSLShaderBuilder.cpp
+++ b/src/gpu/glsl/GrGLSLShaderBuilder.cpp
@@ -158,8 +158,7 @@
     this->versionDecl() = fProgramBuilder->glslCaps()->versionDeclString();
     this->compileAndAppendLayoutQualifiers();
     SkASSERT(visibility);
-    fProgramBuilder->appendUniformDecls((GrGLSLUniformHandler::ShaderVisibility) visibility,
-                                        &this->uniforms());
+    fProgramBuilder->appendUniformDecls((GrShaderFlags) visibility, &this->uniforms());
     this->appendDecls(fInputs, &this->inputs());
     this->appendDecls(fOutputs, &this->outputs());
     this->onFinalize();