Fix data race when concurrent compilation is aborted due to dependency change.

R=marja@chromium.org
BUG=chromium:419189
LOG=N

Review URL: https://codereview.chromium.org/616263003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24361 ce2b1a6d-e550-0410-aec6-3dcde31c8c00