Loading Lookup causes http/tests/cache/post-redirect-get.php to fail

Loading Lookup causes http/tests/cache/post-redirect-get.php to fail
https://bugs.webkit.org/show_bug.cgi?id=139204
<rdar://problem/19120929>

Reviewed by Anders Carlsson.

* WebView/WebView.mm:
(-[WebView _commonInitializationWithFrameName:groupName:]):
(-[WebView _showDictionaryLookupPopup:]):
* WebView/WebViewData.h:
Work around this by delaying the softlinking (and setting up the notification
observer) until we actually need it. There's still an underlying bug, but
this will fix the test.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@176695 268f45cc-cd09-0410-ab3c-d52691b4dbfc