Ensure observers of AppBannerManager are removed on destruction

This CL addresses use-after-free crashes when observers of the
AppBannerManager outlive it.

Bug: 982454
Change-Id: Id9a85f3be7b5129d14774923bd90473679cb48e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1695321
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676011}

chrome/browser/banners/app_banner_manager.cc

@@ -242,7 +242,11 @@ AppBannerManager::AppBannerManager(content::WebContents* web_contents)
   AppBannerSettingsHelper::UpdateFromFieldTrial();
 }
 
-AppBannerManager::~AppBannerManager() { }
+AppBannerManager::~AppBannerManager() {
+  for (Observer& observer : observer_list_)
+    observer.ObserveAppBannerManager(nullptr);
+  CHECK(!observer_list_.might_have_observers());
+}
 
 bool AppBannerManager::CheckIfShouldShowBanner() {
   if (ShouldBypassEngagementChecks())