The problem manifests itself when parsing manages to return something
meaningful in the presence of a stack overflow. This happens because
calling ParserBase::Next() will still return one valid token on stack
overflow, before starting to return invalid tokens.

Take the following input as example:

        a.map(v => v + 1);
              | |
       already   next token
        parsed   (which will be an invalid token
  (identifier)   because of a stack overflow)

The "v" may have been already parsed into a VariableProxy, then if a
stack overflow occurs, next token will be an invalid token (instead
of Token::ARROW), but the parser will return the VariableProxy.

This always happens when lazy-parsing arrow functions, so the position
in the input stream where the the arrow function code ends is known.
This fix adds a check that ensures that parsing ended at the end
position of the arrow function.

BUG=465671
LOG=N

Review URL: https://codereview.chromium.org/1023483003

Cr-Commit-Position: refs/heads/master@{#27325}