Revert of Deprecating CanvasRenderingContext2D.webkitBackingStorePixelRatio (https://codereview.chromium.org/156833002/)

Reason for revert:
Broke a handful of tests - see http://crbug.com/341703

Original issue's description:
> Deprecating CanvasRenderingContext2D.webkitBackingStorePixelRatio
> 
> Should have been deprecated along with webkit[Get|Put]ImageDataHD, but got
> forgotten.  This attribute represents the ratio between HD and non-HD.
> Its value is always 1.
> 
> BUG=277205
> R=senorblanco@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=166648

TBR=senorblanco@chromium.org,junov@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=277205

Review URL: https://codereview.chromium.org/145443010

git-svn-id: svn://svn.chromium.org/blink/trunk@166666 bbb929c8-8fbe-4397-9dbb-9b2b20218538