[WebKit2 iOS]Add support for caret movement for the 3rd party keyboard protocol.
https://bugs.webkit.org/show_bug.cgi?id=135325
<rdar://problem/17682120>

Reviewed by Sam Weinig.

WKContentView now implements moveByOffset to support the
protocol for 3rd party keyboards.

* UIProcess/WebPageProxy.h:
* UIProcess/ios/WKContentViewInteraction.mm:
(-[WKContentView moveByOffset:]):
* UIProcess/ios/WebPageProxyIOS.mm:
(WebKit::WebPageProxy::moveSelectionByOffset):
* WebProcess/WebPage/WebPage.h:
* WebProcess/WebPage/WebPage.messages.in:
* WebProcess/WebPage/ios/WebPageIOS.mm:
(WebKit::WebPage::moveSelectionByOffset):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@171651 268f45cc-cd09-0410-ab3c-d52691b4dbfc