Add virtual dtor to SVGZoomAndPan

Quiets a warning on g++ 4.7.3, per request of Sigbjørn:
https://codereview.chromium.org/227043008/#msg9

(virtual functions but non-virtual dtor in derived class)

NOTRY=true
NOTREECHECK=true

Review URL: https://codereview.chromium.org/226963004

git-svn-id: svn://svn.chromium.org/blink/trunk@171038 bbb929c8-8fbe-4397-9dbb-9b2b20218538