Previously, any AstVisitor subclasses which wanted to make use of
the shared stack overflow checking code needed to depend on Isolate.

With this patch, it will be easy to create a second InitializeAstVisitor
overload taking a stack_limit directly, for use in code that has no
Isolate available (such as code running in the parser).

AstVisitor subclasses which depended upon the isolate() accessor have
been fixed to either have their own isolate_ member or get it from
somewhere else convenient.

Review URL: https://codereview.chromium.org/1387383005

Cr-Commit-Position: refs/heads/master@{#31260}