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

Review URL: https://codereview.chromium.org/156833002

git-svn-id: svn://svn.chromium.org/blink/trunk@166648 bbb929c8-8fbe-4397-9dbb-9b2b20218538