Bug 252415

When a page is backgrounded we try to suspend its process if possible. When the internal setting "Take WebKit:Suspended assertion..." is disabled, this will cause the UI process to drop all of its process assertions on the web content process, effectively making it un-schedulable. This means that pages which may still be active will completely stop. This wasn't an issue before since the web content process would still be schedulable because it was holding the WebKit:Suspended assertion (this is very confusing since the WebKit:Suspended process assertion actually _prevents_ suspension). Instead of dropping all process assertions immediately upon being backgrounded, we should give the page/process some time to be scheduled and if it performs any observable work (e.g. changing document title text, sending a local notification, etc.) we should extend this time to be scheduled.