Add support for reading .gypi files.

tools/gn/scope.h

Index: tools/gn/scope.h
diff --git a/tools/gn/scope.h b/tools/gn/scope.h
index 82e037fe39e101aeb9de70f5930a8351097d1f92..e25283accbeb6f4883a037187b58aa87d9da9666 100644
--- a/tools/gn/scope.h
+++ b/tools/gn/scope.h
@@ -150,9 +150,10 @@ class Scope {
   // the error and returns false if they were.
   bool CheckForUnusedVars(Err* err) const;
 
-  // Returns all values set in the current scope, without going to the parent
-  // scopes.
+  // Returns all key/values or just the keys set in the current scope,
+  // without going to the parent scopes.
   void GetCurrentScopeValues(KeyValueMap* output) const;
+  void GetCurrentScopeKeys(std::vector<base::StringPiece>* output) const;
 
   // Copies this scope's values into the destination. Values from the
   // containing scope(s) (normally shadowed into the current one) will not be