Remove HeapSnapshot.dispose

The method is never called as we terminate the worker when we don't need a snapshot any more.

BUG=None
R=alph@chromium.org

Review URL: https://codereview.chromium.org/358853003

git-svn-id: svn://svn.chromium.org/blink/trunk@176992 bbb929c8-8fbe-4397-9dbb-9b2b20218538