Support partial interface for test support idls

Add implementation to support partial interface for test support idls.

BUG=287635, 261467

Review URL: https://codereview.chromium.org/24053003

git-svn-id: svn://svn.chromium.org/blink/trunk@159635 bbb929c8-8fbe-4397-9dbb-9b2b20218538