IDL compiler: [DoNotCheckSignature] for methods

Also refactoring to introduce |method.is_normal|,
since we divide methods into two groups (via a long boolean condition),
and we don't want to duplicate the test.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/62373007

git-svn-id: svn://svn.chromium.org/blink/trunk@161485 bbb929c8-8fbe-4397-9dbb-9b2b20218538