Clear m_logicallyLastRun when the runs are deleted

If m_logicallyLastRun is not cleared, it ends up pointing to freed
memory, allowing for use-after-free problems.

BUG=344359

Review URL: https://codereview.chromium.org/175553002

git-svn-id: svn://svn.chromium.org/blink/trunk@168146 bbb929c8-8fbe-4397-9dbb-9b2b20218538