Media Galleries: Make sure scan results meet a minimum size requirements.

Also check for overlapping folders to scan.

BUG=161119

Review URL: https://codereview.chromium.org/140313003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248185 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248185