Add APIs for customizing the user agent

Add APIs for customizing the user agent
https://bugs.webkit.org/show_bug.cgi?id=138881
rdar://problem/17233981

Reviewed by Dan Bernstein.

* UIProcess/API/Cocoa/WKWebView.h:
* UIProcess/API/Cocoa/WKWebView.mm:
(-[WKWebView initWithFrame:configuration:]):
(-[WKWebView customUserAgent]):
(-[WKWebView setCustomUserAgent:]):
(-[WKWebView _customUserAgent]):
(-[WKWebView _setCustomUserAgent:]):
* UIProcess/API/Cocoa/WKWebViewConfiguration.h:
* UIProcess/API/Cocoa/WKWebViewConfiguration.mm:
(LazyInitialized::set):
(-[WKWebViewConfiguration copyWithZone:]):
(defaultApplicationNameForUserAgent):
(-[WKWebViewConfiguration applicationNameForUserAgent]):
(-[WKWebViewConfiguration setApplicationNameForUserAgent:]):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@176340 268f45cc-cd09-0410-ab3c-d52691b4dbfc