Remove virtual destructor for Pair class

This destructor was empty and marked as virtual. Removing it avoids creating
an unnecessary vtable since this class is never subclassed nor does it have
any virtual function.

BUG=

Review URL: https://codereview.chromium.org/40883002

git-svn-id: svn://svn.chromium.org/blink/trunk@160481 bbb929c8-8fbe-4397-9dbb-9b2b20218538