added support for push_constant layout

src/sksl/SkSLParser.cpp

Index: src/sksl/SkSLParser.cpp
diff --git a/src/sksl/SkSLParser.cpp b/src/sksl/SkSLParser.cpp
index 324bd41c86c972f39e9ad3714ef4069c90d8c6dd..154989fe84d6c5fbbb388f75c0248d52d0d4b24b 100644
--- a/src/sksl/SkSLParser.cpp
+++ b/src/sksl/SkSLParser.cpp
@@ -526,8 +526,7 @@ int Parser::layoutInt() {
     return -1;
 }
 
-/* LAYOUT LPAREN IDENTIFIER EQ INT_LITERAL (COMMA IDENTIFIER EQ INT_LITERAL)* 
-   RPAREN */
+/* LAYOUT LPAREN IDENTIFIER (EQ INT_LITERAL)? (COMMA IDENTIFIER (EQ INT_LITERAL)?)* RPAREN */
 ASTLayout Parser::layout() {
     int location = -1;
     int binding = -1;
@@ -537,11 +536,12 @@ ASTLayout Parser::layout() {
     bool originUpperLeft = false;
     bool overrideCoverage = false;
     bool blendSupportAllEquations = false;
+    bool pushConstant = false;
     if (this->peek().fKind == Token::LAYOUT) {
         this->nextToken();
         if (!this->expect(Token::LPAREN, "'('")) {
             return ASTLayout(location, binding, index, set, builtin, originUpperLeft,
-                             overrideCoverage, blendSupportAllEquations);
+                             overrideCoverage, blendSupportAllEquations, pushConstant);
         }
         for (;;) {
             Token t = this->nextToken();
@@ -561,6 +561,8 @@ ASTLayout Parser::layout() {
                 overrideCoverage = true;
             } else if (t.fText == "blend_support_all_equations") {
                 blendSupportAllEquations = true;
+            } else if (t.fText == "push_constant") {
+                pushConstant = true;
             } else {
                 this->error(t.fPosition, ("'" + t.fText + 
                                           "' is not a valid layout qualifier").c_str());
@@ -575,7 +577,7 @@ ASTLayout Parser::layout() {
         }
     }
     return ASTLayout(location, binding, index, set, builtin, originUpperLeft, overrideCoverage,
-                     blendSupportAllEquations);
+                     blendSupportAllEquations, pushConstant);
 }
 
 /* layout? (UNIFORM | CONST | IN | OUT | INOUT | LOWP | MEDIUMP | HIGHP | FLAT | NOPERSPECTIVE)* */