Add support for distilling arbitrary URLs in DOM Distiller Viewer.

This adds support for requesting to view arbitrary URLs in the DOM
Distiller viewer. Previously, an entry first had to be added to the
list of articles.

Also fixes the CSP for the viewer to be in the correct format.

BUG=319881
TBR=rsleevi@chromium.org,joi@chromium.org

Review URL: https://codereview.chromium.org/151003006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254458 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254458