This patch parametrizes AstTraversalVisitor by the actual subclass,
in a similar way as AstVisitor is parametrized.  This allows a
subclass to, e.g., override the Visit method and still use the
traversal mechanism.  It also allows the subclass to override the
specific visiting methods, without them being virtual.

This patch also removes AstExpressionVisitor, subsuming its
functionality in AstTraversalVisitor.

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

Review-Url: https://codereview.chromium.org/2169833002
Cr-Commit-Position: refs/heads/master@{#37998}