Killing Login Window not killing all the browser processes

[MasterK0927]

Introduced with #91
Extracted from #92

Currently killing the Login Widget is not killing the browser processes that are running.
But the expected behr is that it should kill all the browser process when we kill the login window (widget).

[MasterK0927]

My investigation till now on this

Currently, when the login widget is closed, it only executes the login success callback but doesn't properly terminate all browser processes.

The issue is with the Current flow:

• BraveStartupBrowserCreatorImpl::Launch starts the process
• LaunchWithAuth creates and shows the login widget
• When login succeeds, CloseAndRunCallback in LoginScreenView is called
• Currently, it just closes the widget and runs the success callback

[MasterK0927]

Upon further investigation, I can confirm that, this issue is coming only after we create the login window, before creating the login window its functioning normal. Need to investigate into the threads management.