Drop [LegacyImplementedInBaseClass] from SVGFitToViewBox IDL interface

Drop [LegacyImplementedInBaseClass] from SVGFitToViewBox IDL interface by
refactoring the code. More code was also moved to SVGFitToViewBox class to
avoid code duplication.

R=haraken, pdr
BUG=344224

Review URL: https://codereview.chromium.org/169033002

git-svn-id: svn://svn.chromium.org/blink/trunk@167269 bbb929c8-8fbe-4397-9dbb-9b2b20218538