[iOS] Fix potential memory leaks.

Fix a UIWindowScene leak in multiwindow. Improve memory management of
overlays.

Bug: 1090263
Change-Id: Ia5e06566635b182fa33c4976c933cebcf0b5f11d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2228654
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774646}

ios/chrome/browser/ui/main/scene_state.h

@@ -68,7 +68,7 @@ typedef NS_ENUM(NSUInteger, SceneActivationLevel) {
 // Window for the associated scene, if any.
 @property(nonatomic, strong) UIWindow* window;
 
-@property(nonatomic, strong) UIWindowScene* scene API_AVAILABLE(ios(13));
+@property(nonatomic, weak) UIWindowScene* scene API_AVAILABLE(ios(13));
 
 @property(nonatomic, strong)
     UISceneConnectionOptions* connectionOptions API_AVAILABLE(ios(13));

ios/chrome/browser/ui/overlays/overlay_presentation_context_impl.h

@@ -183,13 +183,13 @@ class OverlayPresentationContextImpl : public OverlayPresentationContext {
   // The context's delegate.
   __weak id<OverlayPresentationContextImplDelegate> delegate_ = nil;
   // The window in which overlay UI will be presented.
-  UIWindow* window_ = nil;
+  __weak UIWindow* window_ = nil;
   // The UIViewController used as the base for overlays UI displayed using child
   // UIViewControllers.
-  UIViewController* container_view_controller_ = nil;
+  __weak UIViewController* container_view_controller_ = nil;
   // The UIViewController used as the base for overlays displayed using
   // presented UIViewControllers.
-  UIViewController* presentation_context_view_controller_ = nil;
+  __weak UIViewController* presentation_context_view_controller_ = nil;
   // The presentation capabilities of |coordinator_|'s view controller.
   UIPresentationCapabilities presentation_capabilities_ =
       UIPresentationCapabilities::kNone;