Add "." accessors to GN.

tools/gn/scope.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef TOOLS_GN_SCOPE_H_
 #define TOOLS_GN_SCOPE_H_
 
 #include <map>
 #include <set>
 
@@ skipping 68 matching lines @@
   }
 
   // Returns NULL if there's no such value.
   //
   // counts_as_used should be set if the variable is being read in a way that
   // should count for unused variable checking.
   const Value* GetValue(const base::StringPiece& ident,
                         bool counts_as_used);
   const Value* GetValue(const base::StringPiece& ident) const;
 
+  // Returns the requested value as a mutable one if possible. If the value
+  // is not found in a mutable scope, then returns null. Note that the value
+  // could still exist in a const scope, so GetValue() could still return
+  // non-null in this case.
+  //
+  // Say you have a local scope that then refers to the const root scope from
+  // the master build config. You can't change the values from the master
+  // build config (it's read-only so it can be read from multiple threads
+  // without locking). Read-only operations would work on values from the root
+  // scope, but write operations would only work on values in the derived
+  // scope(s).
+  //
+  // Be careful when calling this. It's not normally correct to modify values,
+  // but you should instead do a new Set each time.
+  //
+  // Consider this code:
+  //   a = 5
+  //    {
+  //       a = 6
+  //    }
+  // The 6 should get set on the nested scope rather than modify the value
+  // in the outer one.
+  Value* GetMutableValue(const base::StringPiece& ident, bool counts_as_used);
+
   // Same as GetValue, but if the value exists in a parent scope, we'll copy
   // it to the current scope. If the return value is non-null, the value is
   // guaranteed to be set in the current scope. Generatlly this will be used
   // if the calling code is planning on modifying the value in-place.
   //
   // Since this is used when doing read-modifies, we never count this access
   // as reading the variable, since we assume it will be written to.
   Value* GetValueForcedToCurrentScope(const base::StringPiece& ident,
                                       const ParseNode* set_node);
 
@@ skipping 155 matching lines @@
 
   typedef std::set<ProgrammaticProvider*> ProviderSet;
   ProviderSet programmatic_providers_;
 
   SourceDir source_dir_;
 
   DISALLOW_COPY_AND_ASSIGN(Scope);
 };
 
 #endif  // TOOLS_GN_SCOPE_H_