Add a comment about why we don't implement dir_reader_posix.h on OS X.
Add a comment about why we don't implement dir_reader_posix.h on OS X.

git-svn-id: http://src.chromium.org/svn/trunk/src@41313 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 84ef58dbffe529dced87e31589c8666967e5288c
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser