Expose "suggested filename" for a resource based on its resource response.

Reviewed by Adam Roben.

API pieces:
* WebProcess/InjectedBundle/API/c/WKBundleFrame.cpp:
(WKBundleFrameCopySuggestedFilenameForResourceURL):
* WebProcess/InjectedBundle/API/c/WKBundleFrame.h:
        
Implementation:
* WebProcess/WebPage/WebFrame.cpp:
(WebKit::WebFrame::suggestedFilenameForResourceURL): See if the DocumentLoader has
  a resource for this URL and, if so, return the response's suggested filename.
* WebProcess/WebPage/WebFrame.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76394 268f45cc-cd09-0410-ab3c-d52691b4dbfc