Crash in BraveSidePanelCoordinator::OnTabStripModelChanged

[iefremov]

Crash in BraveSidePanelCoordinator::OnTabStripModelChanged

[simonhong]

checking.

[simonhong]

When SidePanelCoordinator tries to removing observation for de-activated tab's SidePanelRegistry, this crash happened. We call it contextual registry as it is per-tab context registry.
The crash reason is observer list doesn't contain that deactivated tab's `registry at that time.

SidePanelCoordinator starts to observe tab's registry when tab is activated.
and remove observation when tab is de-activated.

There are two points that stop observation.

• SidePanelCoordinator::OnRegistryDestroying() - when observed registry is destroyed
• SidePanelCoordinator::OnTabStripModelChanged() - when tab is deactivated

To make this crash happens, I think active tab's SidePanelRegistry should be destroyed while its tab is still active.
As SidePanelRegistryset as WebContents' UserData, its life cycle is tied with WebContents.
Well.. I think this case would not be happened.