Get rid of special casing for SVGNumber in the bindings generator

Get rid of special casing for SVGNumber in the bindings generator by
adding a new SVGNumber.h header containing a SVGNumber class.

The implementation is now using the SVGNumber type instead of float
and thus no longer requires special handling on bindings side.

Review URL: https://chromiumcodereview.appspot.com/19263002

git-svn-id: svn://svn.chromium.org/blink/trunk@154380 bbb929c8-8fbe-4397-9dbb-9b2b20218538