Remove virtual destructors for CustomElementDefinition and CustomElementRegistry

Remove destructors for CustomElementDefinition and CustomElementRegistry as these
were empty and marked as virtual. This avoids creating unnecessary vtables since
these classes are never subclasses nor do they have any virtual function.

Review URL: https://codereview.chromium.org/41133002

git-svn-id: svn://svn.chromium.org/blink/trunk@160505 bbb929c8-8fbe-4397-9dbb-9b2b20218538