Fix TabManagerTest.TabManagerBasics flakiness with PlzNavigate.

Fix TabManagerTest.TabManagerBasics flakiness with PlzNavigate.

NOTIFICATION_NAV_ENTRY_COMMITTED only checks that a navigation committed (but not complete). This worked fine without PlzNavigate. But with PlzNavigate, timing differences meant that while the renderer was still handling a previous navigation and it got a back/forward command, it would cancel the previous load. The fix is to use the TestNavigationObserver class to wait for navigations to complete.

BUG=504347

[jam, 2016-12-08 06:03:12]

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

[commit-bot: I haz the power, 2016-12-08 06:03:36]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-12-08 07:09:27]

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

[commit-bot: I haz the power, 2016-12-08 07:09:27]

Dry run: This issue passed the CQ dry run.

[jam, 2016-12-08 15:59:32]

Description was changed from

==========
Fix TabManagerTest.TabManagerBasics flakiness with PlzNavigate.

NOTIFICATION_NAV_ENTRY_COMMITTED in general is not a good notification to wait
for, since it only checks that the NavigationController got the requested
navigation, but not that the renderer finished it. Without PlzNavigate, this is
fine. But with PlzNavigate, there were races between previous navigations and
back navigations, and if the former were slow they would cancel out the latter.
The fix is to use the standard TestNavigationObserver class.

BUG=504347
==========

to

==========
Fix TabManagerTest.TabManagerBasics flakiness with PlzNavigate.

NOTIFICATION_NAV_ENTRY_COMMITTED only checks that a navigation committed (but
not complete). This worked fine without PlzNavigate. But with PlzNavigate,
timing differences meant that while the renderer was still handling a previous
navigation and it got a back/forward command, it would cancel the previous load.
The fix is to use the TestNavigationObserver class to wait for navigations to
complete.

BUG=504347
==========

[jam, 2016-12-08 15:59:32]

jam@chromium.org changed reviewers:
+ georgesak@chromium.org