Fix history for reader mode

Fix history for reader mode

When entering reader mode, it was possible to get into a state where
the distilled page would have a pending history entry and would not
have the rest of the history merged. This change moves the history
merge to before a new entry is set in the distilled web contents.

BUG=498576
Committed: https://crrev.com/06c20e773985a26bcadf35623671514e01f2234a
Cr-Commit-Position: refs/heads/master@{#333739}

[mdjones, 2015-06-10 00:58:55]

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

[aruslan, 2015-06-10 01:07:47]

lgtm

[mdjones, 2015-06-10 15:27:57]

The CQ bit was checked by mdjones@chromium.org

[commit-bot: I haz the power, 2015-06-10 15:28:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1171343002/1

[commit-bot: I haz the power, 2015-06-10 15:48:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-10 15:49:09]

Patchset 1 (id:??) landed as
https://crrev.com/06c20e773985a26bcadf35623671514e01f2234a
Cr-Commit-Position: refs/heads/master@{#333739}