Add support for multipage distillation.

Add support for distilling multiple pages, currently support is really
basic and just distills pages sequentially.
BUG=288015
TEST=included
NOTRY=true
R=cjhopman@chromium.org

Review URL: https://codereview.chromium.org/146843010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248867 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248867