[WK2] API::UserContentURLPattern creation functions should return Ref<>

[WK2] API::UserContentURLPattern creation functions should return Ref<>
https://bugs.webkit.org/show_bug.cgi?id=144225

Reviewed by Darin Adler.

Have API::UserContentURLPattern creation functions return Ref<>.
Call-sites are also updated, using and operating on the returned Ref<>
object where possible.

* Shared/API/APIUserContentURLPattern.h:
(API::UserContentURLPattern::create):
* Shared/API/c/WKUserContentURLPattern.cpp:
(WKUserContentURLPatternCreate):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@183463 268f45cc-cd09-0410-ab3c-d52691b4dbfc