This patch applies an adaptation of the Curiously Recurring Template
Pattern to the parser objects.  The result is roughly:

    // Common denominator, needed to avoid cyclic dependency.
    // Instances of this template will end up with very minimal
    // definitions, ideally containing just typedefs.
    template <typename Impl>
    class ParserBaseTraits;

    // The parser base object, which should just implement pure
    // parser behavior.  The Impl parameter is the actual derived
    // class (according to CRTP), which implements impure parser
    // behavior.
    template <typename Impl>
    class ParserBase : public ParserBaseTraits<Impl> { ... };

    // And then, for each parser variant:
    class Parser;

    template <>
    class ParserBaseTraits<Parser> { ... };

    class Parser : public ParserBase<Parser> { ... };

Using the CRTP, we will ultimately achieve two goals:
(1) clean up the traits objects, but most importantly
(2) clearly separate pure/impure parser implementation and facilitate
experimentation with different parser variants.

R=adamk@chromium.org, marja@chromium.org
BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2267663002
Cr-Commit-Position: refs/heads/master@{#38819}