Add JoinExistingSession to the WebContext XPC.
<rdar://problem/13541540> and https://bugs.webkit.org/show_bug.cgi?id=114882

Reviewed by Sam Weinig.

This fixes <keygen> and maybe other things.

* WebProcess/EntryPoint/mac/XPCService/WebContentService.Development/Info.plist:
* WebProcess/EntryPoint/mac/XPCService/WebContentService/Info.plist:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148756 268f45cc-cd09-0410-ab3c-d52691b4dbfc