DomDistillerService is used for ReadingList and ReaderMode.
The requirements for both feature are different.
As DomDistillerService main purpose (cross-platform) is Reader Mode,
this CL creates files for the Reading List distillation.
These files ensure that:
- WebState used for distillation survives at least 10 seconds after the
  end of distillation to allow loading favicon
- There is a 2 seconds delay between end of page load and start of
  distillation.
- Reading List files are in Reading List components.

These 2 requirements are irrelevant for Reader Mode as the page is
already loaded and the favicon is not used.
This CL also restore the behavior of Reader Mode before cl/2529283002.

BUG=648923

Review-Url: https://codereview.chromium.org/2604773002
Cr-Commit-Position: refs/heads/master@{#441060}