Extract the in-memory model from DomDistillerStore

This moves interaction with the in-memory model from the store to its
own class: DomDistillerModel. This also adds two ways to lookup entries
to both the store and the model, GetEntryById and GetEntryByUrl.

BUG=288015
TBR=joi@chromium.org

Review URL: https://codereview.chromium.org/26574002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230025 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230025