Get rid of SVGRect special case from the bindings generator

Get rid of SVGRect special case from the bindings generator by adding a
a typedef to FloatRect in SVGRect.h.

Also use SVGRect type in the implementation API for consistency with
the IDL.

Review URL: https://chromiumcodereview.appspot.com/19096011

git-svn-id: svn://svn.chromium.org/blink/trunk@154204 bbb929c8-8fbe-4397-9dbb-9b2b20218538