Fix leak in debug mirror cache.

When fetching loaded scripts, mirror objects are created and cached.
If the cache is not cleared, it holds script objects alive.

This also fixes a minor issue with script unloading.

R=ulan@chromium.org
BUG=376534
LOG=N

Review URL: https://codereview.chromium.org/296953005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21477 ce2b1a6d-e550-0410-aec6-3dcde31c8c00