[iOS] Add (and use) an explicit -disconnect for TabStripController.

TabStripController relies on -dalloc to stop its various WebStateList
observations. If the WebStateList is destroyed before the dealloc runs,
the non-empty observer DCHECKs are hit. The upcoming change to
WebStateList ownership (see crrev.com/c/1796358) will cause this to
happen, for example.

To fix this, an explicit -disconnect method is added to
TabStripController, and the tab strip coordinator calls it from -stop.

Change-Id: I22c2fc967c0b42c544ab93dce17518e93d272f43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1809298
Commit-Queue: Mark Cogan <marq@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697241}

ios/chrome/browser/ui/tabs/tab_strip_controller.h

@@ -48,6 +48,11 @@
 // Hides or shows the tab strip.
 - (void)hideTabStrip:(BOOL)hidden;
 
+// Preprare the receiver for destruction, disconnecting from all services.
+// It is an error for the receiver to dealloc without this having been called
+// first.
+- (void)disconnect;
+
 @end
 
 #endif  // IOS_CHROME_BROWSER_UI_TABS_TAB_STRIP_CONTROLLER_H_

ios/chrome/browser/ui/tabs/tab_strip_controller.mm

@@ -257,6 +257,9 @@ UIColor* BackgroundColor() {
 @property(nonatomic, readonly, retain) TabStripView* tabStripView;
 @property(nonatomic, readonly, retain) UIButton* buttonNewTab;
 
+// YES if the controller has been disconnected.
+@property(nonatomic) BOOL disconnected;
+
 // Initializes the tab array based on the the entries in the TabModel.  Creates
 // one TabView per Tab and adds it to the tabstrip.  A later call to
 // |-layoutTabs| is needed to properly place the tabs in the correct positions.
@@ -510,10 +513,16 @@ UIColor* BackgroundColor() {
 }
 
 - (void)dealloc {
+  DCHECK(_disconnected);
+}
+
+- (void)disconnect {
   [_tabStripView setDelegate:nil];
   [_tabStripView setLayoutDelegate:nil];
   _allWebStateObservationForwarder.reset();
+  _webStateListFaviconObserver.reset();
   _tabModel.webStateList->RemoveObserver(_webStateListObserver.get());
+  self.disconnected = YES;
 }
 
 - (void)hideTabStrip:(BOOL)hidden {

ios/chrome/browser/ui/tabs/tab_strip_controller_unittest.mm

@@ -126,6 +126,7 @@ class TabStripControllerTest : public PlatformTest {
       return;
 
     [tab_model_ browserStateDestroyed];
+    [controller_ disconnect];
   }
 
   web::WebTaskEnvironment task_environment_;

ios/chrome/browser/ui/tabs/tab_strip_legacy_coordinator.mm

@@ -88,6 +88,7 @@
 
 - (void)stop {
   self.started = NO;
+  [self.tabStripController disconnect];
   self.tabStripController = nil;
   self.dispatcher = nil;
   self.tabModel = nil;