Remove overeager DCHECK in SnapshotController.

Remove overeager DCHECK in SnapshotController.
The DCHECK is not needed because indeed the state of the controller can be 'kReady' as a result
of calling Reset() on it. The scenario can be as following:
- snapshot started
- page is navigated while snapshot is in progress. This causes immediate Reset() on SnapshotController.
- This may or may not cancel snapshot, but ultimately the RecentTabHelper will report that snapshot is completed.
  If the Reset() above already happened, the DCHECK will fire. It should not, since
  this is normal conditions.

Also updated RecentTabHelper to stop posted tasks related to previous snapshot when page is re-navigated.
Removed chack for IsErrorPage() since it will be done later by checking CanSavePage anyways.

BUG=616570
Committed: https://crrev.com/b9c88f7c6eb09ba85ea6fce0316a9f7639fdc40b
Cr-Commit-Position: refs/heads/master@{#397535}

[Dmitry Titov, 2016-06-02 00:38:39]

dimich@chromium.org changed reviewers:
+ dewittj@chromium.org

[Dmitry Titov, 2016-06-02 00:38:40]

ptal

[dewittj, 2016-06-02 18:06:52]

lgtm, is there an easy test to write that exercises this bug?

[Dmitry Titov, 2016-06-02 20:37:25]

Added a simple test.

[Dmitry Titov, 2016-06-02 20:37:29]

The CQ bit was checked by dimich@chromium.org

[Dmitry Titov, 2016-06-02 20:37:30]

The patchset sent to the CQ was uploaded after l-g-t-m from dewittj@chromium.org
Link to the patchset: https://codereview.chromium.org/2030793002/#ps40001
(title: "added test")

[commit-bot: I haz the power, 2016-06-02 20:38:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/2030793002/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/2030793002/40001

[commit-bot: I haz the power, 2016-06-02 22:47:41]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2016-06-02 22:49:38]

Description was changed from

==========
Remove overeager DCHECK in SnapshotController.
The DCHECK is not needed because indeed the state of the controller can be
'kReady' as a result
of calling Reset() on it. The scenario can be as following:
- snapshot started
- page is navigated while snapshot is in progress. This causes immediate Reset()
on SnapshotController.
- This may or may not cancel snapshot, but ultimately the RecentTabHelper will
report that snapshot is completed.
  If the Reset() above already happened, the DCHECK will fire. It should not,
since
  this is normal conditions.

Also updated RecentTabHelper to stop posted tasks related to previous snapshot
when page is re-navigated.
Removed chack for IsErrorPage() since it will be done later by checking
CanSavePage anyways.

BUG=616570
==========

to

==========
Remove overeager DCHECK in SnapshotController.
The DCHECK is not needed because indeed the state of the controller can be
'kReady' as a result
of calling Reset() on it. The scenario can be as following:
- snapshot started
- page is navigated while snapshot is in progress. This causes immediate Reset()
on SnapshotController.
- This may or may not cancel snapshot, but ultimately the RecentTabHelper will
report that snapshot is completed.
  If the Reset() above already happened, the DCHECK will fire. It should not,
since
  this is normal conditions.

Also updated RecentTabHelper to stop posted tasks related to previous snapshot
when page is re-navigated.
Removed chack for IsErrorPage() since it will be done later by checking
CanSavePage anyways.

BUG=616570

Committed: https://crrev.com/b9c88f7c6eb09ba85ea6fce0316a9f7639fdc40b
Cr-Commit-Position: refs/heads/master@{#397535}
==========

[commit-bot: I haz the power, 2016-06-02 22:49:39]

Patchset 3 (id:??) landed as
https://crrev.com/b9c88f7c6eb09ba85ea6fce0316a9f7639fdc40b
Cr-Commit-Position: refs/heads/master@{#397535}