[Offline pages] Properly informing observers when offline page is deleted

[Offline pages] Properly informing observers when offline page is deleted

Currently the OfflinePageModel::Observer::OfflinePageDeleted is only
issued when a page is marked for deletion (for bookmark delete undo).
This page also issues it when the offline copy is directly removed
and takes care that it not duplicated when a page has been previously
marked for deletion.

BUG=559100
R=jianli@chromium.org

Committed: https://crrev.com/eb74216daa0f0606140d53cf36494b51b2b2dc8f
Cr-Commit-Position: refs/heads/master@{#360959}

[fgorski, 2015-11-20 22:18:32]

PTAL

[jianli, 2015-11-20 22:28:05]

lgtm

[fgorski, 2015-11-20 23:04:31]

The CQ bit was checked by fgorski@chromium.org

[commit-bot: I haz the power, 2015-11-20 23:08:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1460413002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1460413002/1

[commit-bot: I haz the power, 2015-11-21 00:19:50]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-21 00:20:59]

Patchset 1 (id:??) landed as
https://crrev.com/eb74216daa0f0606140d53cf36494b51b2b2dc8f
Cr-Commit-Position: refs/heads/master@{#360959}