Added scene locking capability.

Added two functions, WKCoordinatedSceneLockState and WKCoordinatedSceneUnlockState.
Those should be called e.g. by responding to didStartProvisionalLoadForFrame or
didSameDocumentNavigationForFrame, to ensure that the state of the scene stays
the same as it was before the load until it is unlocked.

Reviewed by Marcelo Lira.
Added API tests for scene locking.

Reviewed by: Hugo Lima