Allow --enable-app-list-folder in about:flags in Linux Aura.

BUG=343362

Review URL: https://codereview.chromium.org/165923002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251633 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251633