Media Galleries: Skip hidden files on scans.

Also do not follow symlinks.

BUG=161119

Review URL: https://codereview.chromium.org/134183005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248155 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248155