Cancel dialogs for Tabs when they are removed from their TabModels.

ios/chrome/browser/ui/browser_view_controller.mm

Index: ios/chrome/browser/ui/browser_view_controller.mm
diff --git a/ios/chrome/browser/ui/browser_view_controller.mm b/ios/chrome/browser/ui/browser_view_controller.mm
index 6dfaffb36c032fe17b0500efa6baeb8e678b3022..29a430b2bf7b37f8e56dace02223da58952e132f 100644
--- a/ios/chrome/browser/ui/browser_view_controller.mm
+++ b/ios/chrome/browser/ui/browser_view_controller.mm
@@ -4632,6 +4632,9 @@ - (void)tabModel:(TabModel*)model
          atIndex:(NSUInteger)index {
   [self uninstallDelegatesForTab:tab];
 
+  // Cancel dialogs for |tab|'s WebState.
+  [self.dialogPresenter cancelDialogForWebState:tab.webState];

[sdefresne, 2017/06/10 16:43:40]

There is the following DECHK in -cancelDialogForWebState:

  DCHECK_NE(webState, self.presentedDialogWebState);

Reading the code of DialogPresenter, I see nothing that a WebState that is going
to be closed cannot show a dialog. Thus, I think it is possible for that DCHECK
to fail. Should we change that function to support webState ==
self.presentedDialogWebState, or add another function that do support this
state?

+
   // Remove stored native controllers for the tab.
   [_nativeControllersForTabIDs removeObjectForKey:tab.tabId];