Add support for lookup on iOS.

Add support for lookup on iOS.
https://bugs.webkit.org/show_bug.cgi?id=144342
rdar://problem/19994090

Reviewed by Tim Horton.

* UIProcess/WebPageProxy.h:
* UIProcess/ios/WKContentViewInteraction.mm:
(-[WKContentView _lookup:]):
(-[WKContentView canPerformAction:withSender:]):
* UIProcess/ios/WebPageProxyIOS.mm:
(WebKit::WebPageProxy::getLookupContextAtPoint):
* WebProcess/WebPage/WebPage.h:
* WebProcess/WebPage/WebPage.messages.in:
* WebProcess/WebPage/ios/WebPageIOS.mm:
(WebKit::WebPage::getLookupContextAtPoint):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@183503 268f45cc-cd09-0410-ab3c-d52691b4dbfc