Store page no for distilled pages undergoing distillation.

In order to support distillation of previous pages and enable
meaningful incremental updates for viewers, distiller needs
to maintain page number information for pages under distillation.
This information will be used to add support for incremental updates
and distilling previous pages of an article.

BUG=288015
TEST=Covered by existing tests + added tests for failure   and 
     page limit for distiller.

Review URL: https://codereview.chromium.org/130543003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251546 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251546