Adding -[AddAccountSigninCoordinator interruptWithAction:completion:]

Adding implementation for  -[AddAccountSigninCoordinator
interruptWithAction:completion:], and updating SigninCoordinator
to call this method when the sign-in should be canceled.

Also, when -[AddAccountSigninCoordinator stop] is called, the following
instance variable should be nil:
 - signinCompletion
 - identityInteractionManager
 - userSigninCoordinator
 - alertCoordinator
Those variables should set to nil in -[AddAccountSigninCoordinator
runCompletionCallbackWithSigninResult:identity:].

Adding [super stop] and [super start] in start and stop methods of
AddAccountSigninCoordinator.

Bug: 971989
Change-Id: Icdb209a2edab58bd3bc42472534a7f3d4c7b3299
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2046529
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741110}

• ios/chrome/browser/ui/authentication/signin/add_account_signin/add_account_signin_coordinator.mm[diff]
• ios/chrome/browser/ui/signin_interaction/signin_interaction_coordinator.mm[diff]