IDL compiler: constants and [CallWith] attributes in partial interfaces

Straightforward.

Also:
* removes test for [Custom] attributes in partial interfaces, as no real use

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/131743011

git-svn-id: svn://svn.chromium.org/blink/trunk@165269 bbb929c8-8fbe-4397-9dbb-9b2b20218538