Contrary to AstVisitor, which does not implement any traversal logic,
AstTraversalVisitor provides default implementations for each Visit*
function which walk through the AST. It is intended to be used as a base
class for visitors which are only interested in a small portion of the
AST.

R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/1963243003
Cr-Commit-Position: refs/heads/master@{#36283}