added support for push_constant layout

src/sksl/ir/SkSLModifiers.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_MODIFIERS
 #define SKSL_MODIFIERS
 
@@ skipping 20 matching lines @@
     };
 
     Modifiers(const ASTModifiers& modifiers)
     : fLayout(modifiers.fLayout)
     , fFlags(modifiers.fFlags) {}
 
     Modifiers(Layout& layout, int flags)
     : fLayout(layout)
     , fFlags(flags) {}
 
+    Modifiers()
+    : fLayout(Layout())
+    , fFlags(0) {}
+
     std::string description() const {
         std::string result = fLayout.description();
         if (fFlags & kUniform_Flag) {
             result += "uniform ";
         }
         if (fFlags & kConst_Flag) {
             result += "const ";
         }
         if (fFlags & kLowp_Flag) {
             result += "lowp ";
@@ skipping 30 matching lines @@
         return !(*this == other);
     }
 
     Layout fLayout;
     int fFlags;
 };
 
 } // namespace
 
 #endif