added support for push_constant layout

src/sksl/ast/SkSLASTLayout.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
  
 #ifndef SKSL_ASTLAYOUT
 #define SKSL_ASTLAYOUT
 
 #include "SkSLASTNode.h"
 #include "SkSLUtil.h"
 
 namespace SkSL {
 
 /**
  * Represents a layout block appearing before a variable declaration, as in:
  *
  * layout (location = 0) int x;
  */
 struct ASTLayout : public ASTNode {
-    // For all parameters, a -1 means no value
+    // For int parameters, a -1 means no value
     ASTLayout(int location, int binding, int index, int set, int builtin, bool originUpperLeft,
-              bool overrideCoverage, bool blendSupportAllEquations)
+              bool overrideCoverage, bool blendSupportAllEquations, bool pushConstant)
     : fLocation(location)
     , fBinding(binding)
     , fIndex(index)
     , fSet(set)
     , fBuiltin(builtin)
     , fOriginUpperLeft(originUpperLeft)
     , fOverrideCoverage(overrideCoverage)
-    , fBlendSupportAllEquations(blendSupportAllEquations) {}
+    , fBlendSupportAllEquations(blendSupportAllEquations)
+    , fPushConstant(pushConstant) {}
 
     std::string description() const {
         std::string result;
         std::string separator;
         if (fLocation >= 0) {
             result += separator + "location = " + to_string(fLocation);
             separator = ", ";
         }
         if (fBinding >= 0) {
             result += separator + "binding = " + to_string(fBinding);
@@ skipping 16 matching lines @@
             separator = ", ";
         }
         if (fOverrideCoverage) {
             result += separator + "override_coverage";
             separator = ", ";
         }
         if (fBlendSupportAllEquations) {
             result += separator + "blend_support_all_equations";
             separator = ", ";
         }
+        if (fPushConstant) {
+            result += separator + "push_constant";
+            separator = ", ";
+        }
         if (result.length() > 0) {
             result = "layout (" + result + ")";
         }
         return result;
     }
 
     const int fLocation;
     const int fBinding;
     const int fIndex;
     const int fSet;
     const int fBuiltin;
     const bool fOriginUpperLeft;
     const bool fOverrideCoverage;
     const bool fBlendSupportAllEquations;
+    const bool fPushConstant;
 };
 
 } // namespace
 
 #endif