added support for push_constant layout

src/sksl/ast/SkSLASTLayout.h

Index: src/sksl/ast/SkSLASTLayout.h
diff --git a/src/sksl/ast/SkSLASTLayout.h b/src/sksl/ast/SkSLASTLayout.h
index 515eb2bdbbbbca680eb4830ccb78b1b6ba5ba56f..2aec31e4df1f78e9afa2aa288c192dbef637f29f 100644
--- a/src/sksl/ast/SkSLASTLayout.h
+++ b/src/sksl/ast/SkSLASTLayout.h
@@ -19,9 +19,9 @@ namespace SkSL {
  * 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)
@@ -29,7 +29,8 @@ struct ASTLayout : public ASTNode {
     , fBuiltin(builtin)
     , fOriginUpperLeft(originUpperLeft)
     , fOverrideCoverage(overrideCoverage)
-    , fBlendSupportAllEquations(blendSupportAllEquations) {}
+    , fBlendSupportAllEquations(blendSupportAllEquations)
+    , fPushConstant(pushConstant) {}
 
     std::string description() const {
         std::string result;
@@ -66,6 +67,10 @@ struct ASTLayout : public ASTNode {
             result += separator + "blend_support_all_equations";
             separator = ", ";
         }
+        if (fPushConstant) {
+            result += separator + "push_constant";
+            separator = ", ";
+        }
         if (result.length() > 0) {
             result = "layout (" + result + ")";
         }
@@ -80,6 +85,7 @@ struct ASTLayout : public ASTNode {
     const bool fOriginUpperLeft;
     const bool fOverrideCoverage;
     const bool fBlendSupportAllEquations;
+    const bool fPushConstant;
 };
 
 } // namespace