WK2 icon database should be able to get a CGImage of a specific size

Reviewed by Anders Carlsson.

../WebCore: 

* platform/graphics/BitmapImage.h:
* platform/graphics/Image.h:
(WebCore::Image::getFirstCGImageRefOfSize):
        
* platform/graphics/cg/ImageCG.cpp:
(WebCore::BitmapImage::getFirstCGImageRefOfSize): Walk the frames of the image until reaching the
  first frame of the requested size.

../WebKit2: 

* UIProcess/API/C/cg/WKIconDatabaseCG.cpp:
(WKIconDatabaseTryGetCGImageForURL): Change this API to take a requested size, and find the first matching
  CGImage in the icon.
* UIProcess/API/C/cg/WKIconDatabaseCG.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@83091 268f45cc-cd09-0410-ab3c-d52691b4dbfc