[giow] (1) Define getContext('2d', {alpha:false}).
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23515
Affected topics: Canvas, DOM APIs

git-svn-id: http://svn.whatwg.org/webapps@8755 340c8d12-0b0e-0410-8428-c7bf67bfef74