Add -[WKWebView loadData:MIMEType:textEncodingName:baseURL:]
https://bugs.webkit.org/show_bug.cgi?id=138929
<rdar://problem/18683802>

Reviewed by Dan Bernstein.

Also, re-implement -[WKWebView loadHTMLString:baseURL:] in terms of this newly added function
and assume a UTF-8 encoding. This matches what Legacy WebKit does.

* UIProcess/API/Cocoa/WKWebView.h:
* UIProcess/API/Cocoa/WKWebView.mm:
(-[WKWebView loadHTMLString:baseURL:]):
(-[WKWebView loadData:MIMEType:textEncodingName:baseURL:]):
* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::loadData):
* UIProcess/WebPageProxy.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@176408 268f45cc-cd09-0410-ab3c-d52691b4dbfc