As the lifespan of the DistillerPage is shorter than the one needed by the
WebState to download the favicon, the WebState needs to outlive the
DistillerPage.
As the favicon download should not impact too much the distillation time, the
next distillation should be launch before the previous one is finished.
This Dispatcher will makes sure the WebState survive long enough to download
the Favicon after it is returned.

BUG=664988

Review-Url: https://codereview.chromium.org/2529283002
Cr-Commit-Position: refs/heads/master@{#440065}