[SB Delayed Warnings] Fix crash when the old RVH is shut down

Bug: 1130092
Change-Id: I50224da935beb436f2828ef8047435882a044452
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419389Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809182}

chrome/browser/safe_browsing/user_interaction_observer.cc

@@ -143,10 +143,13 @@ SafeBrowsingUserInteractionObserver::FromWebContents(
 void SafeBrowsingUserInteractionObserver::RenderViewHostChanged(
     content::RenderViewHost* old_host,
     content::RenderViewHost* new_host) {
-  old_host->GetWidget()->RemoveKeyPressEventCallback(key_press_callback_);
+  // |old_host| can be nullptr if the old RVH was shut down.
+  if (old_host)
+    old_host->GetWidget()->RemoveKeyPressEventCallback(key_press_callback_);
   new_host->GetWidget()->AddKeyPressEventCallback(key_press_callback_);
 
-  old_host->GetWidget()->RemoveMouseEventCallback(mouse_event_callback_);
+  if (old_host)
+    old_host->GetWidget()->RemoveMouseEventCallback(mouse_event_callback_);
   new_host->GetWidget()->AddMouseEventCallback(mouse_event_callback_);
 }