IDL compiler: merge and simplify EventHandler non-callable value check

Followup to:
IDL compiler: move EventHandler null check out of generated bindings
https://codereview.chromium.org/32323012/

Cleanup, merges two if checks into one in several places.
Also adds check for isAttribute one place where we're assuming it.

BUG=239771

Review URL: https://codereview.chromium.org/34393005

git-svn-id: svn://svn.chromium.org/blink/trunk@160304 bbb929c8-8fbe-4397-9dbb-9b2b20218538