Some additions to support symlinks better on platforms that support them.

      BUG=none
TEST=Ran new unit test, passed trybots.

Review URL: http://codereview.chromium.org/5349007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@67631 0039d316-1c4b-4281-b951-d872f2087c98