[iOS] Notify BrowserObservers of TestBrowser destruction.

This allows BrowserObservers to remove themselves in tests to avoid
the checks_empty=true DCHECK in the ObserverList.

Bug: none
Change-Id: I3d87782a29c72b800333cd9eb4b79244daf45adb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1598874
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657370}

ios/chrome/browser/main/test_browser.mm

@@ -21,7 +21,11 @@ TestBrowser::TestBrowser(ios::ChromeBrowserState* browser_state,
                          WebStateList* web_state_list)
     : browser_state_(browser_state), web_state_list_(web_state_list) {}
 
-TestBrowser::~TestBrowser() {}
+TestBrowser::~TestBrowser() {
+  for (auto& observer : observers_) {
+    observer.BrowserDestroyed(this);
+  }
+}
 
 ios::ChromeBrowserState* TestBrowser::GetBrowserState() const {
   return browser_state_;