Protect against unknown idl types

In case there is a Conditional for a type in the method declaration and the same type is being used in the parameter list, currently
the idl can't be processed. Fix this by skipping include generation for unknown idl types.

Review URL: https://codereview.chromium.org/350033005

git-svn-id: svn://svn.chromium.org/blink/trunk@177315 bbb929c8-8fbe-4397-9dbb-9b2b20218538