Add RemoveObserver to CookieControlsHandler destructor

OnJavascriptDisallowed does not get called to remove the observer if
the incognito ntp tab is closed, which can result in a crash (see bug).
To fix this, add RemoveObserver call to the destructor.

Bug: 1058769
Change-Id: I18413d0ae54593adc032aa017f39f5787bd5bc31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088713
Commit-Queue: Ehimare Okoyomon <eokoyomon@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747692}

chrome/browser/ui/webui/ntp/cookie_controls_handler.cc

@@ -17,11 +17,12 @@ static const char* kSettingsIcon = "cr:settings_icon";
 static const char* kPolicyIcon = "cr20:domain";
 }  // namespace
 
-CookieControlsHandler::CookieControlsHandler(Profile* profile) {
-  service_ = CookieControlsServiceFactory::GetForProfile(profile);
-}
+CookieControlsHandler::CookieControlsHandler(Profile* profile)
+    : service_(CookieControlsServiceFactory::GetForProfile(profile)) {}
 
-CookieControlsHandler::~CookieControlsHandler() = default;
+CookieControlsHandler::~CookieControlsHandler() {
+  service_->RemoveObserver(this);
+}
 
 void CookieControlsHandler::RegisterMessages() {
   web_ui()->RegisterMessageCallback(