There are entries in the DEPS file where two folders uses the same
git URL (ie. freetype2).  This doesn't work well with git caches because
each task will run on it's own and might try to clobber on top of each other.

This adds another field in a WorkItem which is a list of resources.  When the
work queue is flushed, it has to make sure that none of a newly added workitem
has any resource conflicts.

BUG=618124

Review-Url: https://codereview.chromium.org/2049583003