[iOS] Update DialogPresenterDelegate and its implementation in BVC.

This CL updates DialogPresenterDelegate's
dialogPresenterDelegateIsPresenting property to follow delegate naming
guidelines.  Additionally, rather than storing this value as a property,
this CL updates the implementation to calculate on the fly whether BVC
or any of its children are modally presenting a UIViewController.

Bug: 893660
TEST: Follow repro steps in c#0 on crbug.com/893660
Change-Id: Id75e4e72bac6365d47caed11317c8a6a11a08dbc
Reviewed-on: https://chromium-review.googlesource.com/c/1492364
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636655}
4 files changed