Keep the original contents when showing the distilled one

Keep the original contents when showing the distilled one

This is one of the possible M-45 solutions for
http://crbug.com/504191. Unfortunately, it introduces
at most one non-leaking but unnecessary WebContents
per tab being held after the distillation was used on
the tab.

The proper solution discussed with mdjones@ involves
discarding the cached original page WebContents on any
navigation (except going back) and using it for the rapid
back-press.

Given that the direction for M-46 is to switch the Reader
Mode view to be an overlay (like CS) it is understood to be
unnecessary and unfeasible to complicate M-45, as this code
is going to be rewritten anyway.

BUG=504191
Committed: https://crrev.com/78c6498159eb896a150d50c502747d63915d107c
Cr-Commit-Position: refs/heads/master@{#338380}

[aruslan, 2015-07-08 21:59:37]

aruslan@chromium.org changed reviewers:
+ mdjones@chromium.org

[aruslan, 2015-07-08 21:59:37]

Matthew, PTAL, thanks!

[aruslan, 2015-07-08 22:00:06]

The CQ bit was checked by aruslan@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-08 22:00:27]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1229713002/40001

[commit-bot: I haz the power, 2015-07-08 22:45:25]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-08 22:45:26]

Dry run: This issue passed the CQ dry run.

[mdjones, 2015-07-09 00:56:04]

As we discussed, this CL allows a third WebContents to exist if a user navigates
off of a distilled page via the toolbar. This situation can be handled in a
later CL. lgtm

[aruslan, 2015-07-10 20:13:39]

aruslan@chromium.org changed reviewers:
+ cjhopman@chromium.org

[aruslan, 2015-07-10 20:13:52]

Chris PTAL, thanks!

[cjhopman, 2015-07-10 21:35:06]

lgtm

[aruslan, 2015-07-10 21:38:46]

The CQ bit was checked by aruslan@chromium.org

[commit-bot: I haz the power, 2015-07-10 21:39:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1229713002/40001

[commit-bot: I haz the power, 2015-07-10 22:16:50]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-07-10 22:17:38]

Patchset 3 (id:??) landed as
https://crrev.com/78c6498159eb896a150d50c502747d63915d107c
Cr-Commit-Position: refs/heads/master@{#338380}