Do not clear mirror cache when fetching loaded scripts.

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

Review URL: https://codereview.chromium.org/309313002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21737 ce2b1a6d-e550-0410-aec6-3dcde31c8c00