Remove [LegacyTreatAsPartialInterface] from SVGZoomAndPan

implemented SVG interfaces: 1/5

This has a good example of the problems using static member functions
for implemented interfaces causes: this fixes a hierarchy violation
where interface and implementation for derived class SVGViewSpec
is defined in the base class SVGZoomAndPan !

R=haraken
BUG=360435

Review URL: https://codereview.chromium.org/227043008

git-svn-id: svn://svn.chromium.org/blink/trunk@171022 bbb929c8-8fbe-4397-9dbb-9b2b20218538