Fix a wrong code example in a comment.

We should not add "void" to destructors.

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/119143005

git-svn-id: svn://svn.chromium.org/blink/trunk@164491 bbb929c8-8fbe-4397-9dbb-9b2b20218538