Media Galleries: Add chrome.mediaGalleries.addUserSelectedFolder().

BUG=318411

Review URL: https://codereview.chromium.org/93643002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244128 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244128