Drop [LegacyImplementedInBaseClass] from SVGZoomAndSpan IDL interface

Drop [LegacyImplementedInBaseClass] from SVGZoomAndSpan IDL interface by
refactoring the code a little bit.

Also have SVGViewSpec implement to SVGZoomAndSpan IDL interface instead of
duplicating the SVGZoomAndSpan API. This was done because unlike other
interfaces, SVGViewSpec's zoomAndSpan attribute setter raises an exception.
However, this can be handled in SVGZoomAndSpan class by overloading the
setZoomAndSpan method.

R=haraken, pdr
BUG=344224

Review URL: https://codereview.chromium.org/168923002

git-svn-id: svn://svn.chromium.org/blink/trunk@167260 bbb929c8-8fbe-4397-9dbb-9b2b20218538