DRT and WRT should have HiDPI testing capabilities

Reviewed by Darin Adler.

Source/WebKit/mac: 

New method _backingScaleFactor matches SPI of a similar name that I added 
recently to WKPage in WebKit2. Equivalent functionality is needed for 
DumpRenderTree's HiDPI support.
* WebView/WebView.mm:
(-[WebView _backingScaleFactor]):
* WebView/WebViewPrivate.h:

Tools: 

New layoutTestController function setBackingScaleFactor takes a double for a scale 
factor and a callback function to call once the backing scale factor has 
successfully been set for the view.
* DumpRenderTree/LayoutTestController.cpp:
(setBackingScaleFactorCallback):
(LayoutTestController::staticFunctions):

On Mac, setBackingScaleFactor calls WebView _setCustomBackingScaleFactor. Other 
platforms do nothing.
* DumpRenderTree/LayoutTestController.h:
* DumpRenderTree/mac/LayoutTestControllerMac.mm:
(LayoutTestController::setBackingScaleFactor):
* DumpRenderTree/gtk/LayoutTestControllerGtk.cpp:
(L...