Fix HeapSnapshotView leaks

HeapSnapshotView should be unsubscribed from events it has been subscribed to, otherwise it will leak when corresponding profile is removed.

BUG=None

Review URL: https://codereview.chromium.org/208053003

git-svn-id: svn://svn.chromium.org/blink/trunk@169738 bbb929c8-8fbe-4397-9dbb-9b2b20218538