Try saving pinned tabs on OnBrowserRemoved

Bug:71939

PinnedTabService is observing two events:
NOTIFICATION_CLOSE_ALL_BROWSERS_REQUEST and BrowserList::OnBrowserClosing.

The first is when users want to shut down entire app and
the latter is when users close a browser at once even if it has multiple tabs.

But the bug is edge case which doesn't belong to any case above for now.
When tab strip is empty, BrowserListObserver::OnBrowserClosing is not called even
this will end up with closing the browser.

So try saving pinned tabs again on BrowserListObserver::OnBrowserRemoved.

Change-Id: Id73891b9509918bca3b1dcaf0b34cc32a7a687a7
Test: PinnedTabServiceBrowserTest.*
Reviewed-on: https://chromium-review.googlesource.com/892539
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537442}
4 files changed