[iOS] Remove observer in IOSChromeControllerClient destructor

App was crashing when trying to navigate from an interstitial page
(forward, backward, back to safety). Remove WebStateObserver in
both the destructor and WebStateDestroyed so that both navigations
and closing the page are covered.

Bug: 1066445
Change-Id: Iace4d26edbaec1ccc26a9b6c410a4de827c5be7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2130777
Commit-Queue: Livvie Lin <livvielin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755505}

ios/chrome/browser/interstitials/ios_chrome_controller_client.mm

@@ -32,7 +32,11 @@ IOSChromeControllerClient::IOSChromeControllerClient(
   web_state_->AddObserver(this);
 }
 
-IOSChromeControllerClient::~IOSChromeControllerClient() {}
+IOSChromeControllerClient::~IOSChromeControllerClient() {
+  if (web_state_) {
+    web_state_->RemoveObserver(this);
+  }
+}
 
 void IOSChromeControllerClient::SetWebInterstitial(
     web::WebInterstitial* web_interstitial) {