IDL parser: clean up cached lexer table updating code

Follow-up to
  https://codereview.chromium.org/425953002/
addressing some late comments.

BUG=397909

Review URL: https://codereview.chromium.org/462613004

git-svn-id: svn://svn.chromium.org/blink/trunk@179971 bbb929c8-8fbe-4397-9dbb-9b2b20218538