Reafactored Lexer class to allow chaining. The full chain when parsing #if directive looks like this: Preprocessor -> MacroExpander -> DirectiveHandler -> MacroExpander -> DefinedFilter -> Tokenizer. This chain dynamically changes depending on the current context.

Also added an incomplete implementation of #if handling and ExpressionParser to illustrate how this design is supposed to work.
Review URL: https://codereview.appspot.com/6203060

git-svn-id: https://angleproject.googlecode.com/svn/trunk@1084 736b8ea6-26fd-11df-bfd4-992fa37f6226