[ios] Properly sets the Incognito Browser after closing the last incognito tab.

The code was incorrectly passing the main (non-Incognito) Browser to the
TabGrid.

BUG=1043935

Change-Id: I7c5348c4424a2c199194455b2286eec9efdaabe1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2012088
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Mohammad Refaat <mrefaat@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733772}

ios/chrome/browser/ui/main/scene_controller.mm

@@ -1287,8 +1287,8 @@ enum class EnterTabSwitcherSnapshotResult {
   BOOL otrBVCIsCurrent = (self.interfaceProvider.mainInterface.bvc ==
                           self.interfaceProvider.incognitoInterface.bvc);
 
-  // Clear the OTR tab model and notify the _tabSwitcher that its otrBVC will
-  // be destroyed.
+  // Clear the Incognito Browser and notify the _tabSwitcher that its otrBrowser
+  // will be destroyed.
   [self.mainController.tabSwitcher setOtrBrowser:nil];
 
   [self.mainController.browserViewWrangler destroyAndRebuildIncognitoBrowser];
@@ -1297,9 +1297,10 @@ enum class EnterTabSwitcherSnapshotResult {
     [self activateBVCAndMakeCurrentBVCPrimary];
   }
 
-  // Always set the new otr tab model for the tablet or grid switcher.
-  // Notify the _tabSwitcher with the new otrBVC.
-  [self.mainController.tabSwitcher setOtrBrowser:self.mainInterface.browser];
+  // Always set the new otr Browser for the tablet or grid switcher.
+  // Notify the _tabSwitcher with the new Incognito Browser.
+  [self.mainController.tabSwitcher
+      setOtrBrowser:self.incognitoInterface.browser];
 
   // This seems the best place to deem the destroying and rebuilding the
   // incognito browser state to be completed.