Add gl_SampleMask functionality to fragment builders

src/gpu/glsl/GrGLSLShaderBuilder.cpp

Index: src/gpu/glsl/GrGLSLShaderBuilder.cpp
diff --git a/src/gpu/glsl/GrGLSLShaderBuilder.cpp b/src/gpu/glsl/GrGLSLShaderBuilder.cpp
index c94a85534f7fb7b3ec767ecced979b2a50c7af30..30bf86d2106c98ac3960865d3f6373e601dd2e92 100644
--- a/src/gpu/glsl/GrGLSLShaderBuilder.cpp
+++ b/src/gpu/glsl/GrGLSLShaderBuilder.cpp
@@ -112,11 +112,13 @@ void GrGLSLShaderBuilder::appendTextureLookupAndModulate(const char* modulation,
     this->codeAppend((GrGLSLExpr4(modulation) * GrGLSLExpr4(lookup)).c_str());
 }
 
-void GrGLSLShaderBuilder::addFeature(uint32_t featureBit, const char* extensionName) {
-    if (!(featureBit & fFeaturesAddedMask)) {
-        this->extensions().appendf("#extension %s: require\n", extensionName);
-        fFeaturesAddedMask |= featureBit;
+bool GrGLSLShaderBuilder::addFeature(uint32_t featureBit, const char* extensionName) {
+    if (featureBit & fFeaturesAddedMask) {
+        return false;
     }
+    this->extensions().appendf("#extension %s: require\n", extensionName);
+    fFeaturesAddedMask |= featureBit;
+    return true;
 }
 
 void GrGLSLShaderBuilder::appendDecls(const VarArray& vars, SkString* out) const {