[giow] (0) ImageData objects now expose an explicit pixel density, enabling them to be converted to BitmapImage objects correctly.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=21329
Affected topics: Canvas, DOM APIs

git-svn-id: http://svn.whatwg.org/webapps@7849 340c8d12-0b0e-0410-8428-c7bf67bfef74