Use an MutationObserver to check when a password form disappears after XHR

components/autofill/content/renderer/password_autofill_agent.h

Index: components/autofill/content/renderer/password_autofill_agent.h
diff --git a/components/autofill/content/renderer/password_autofill_agent.h b/components/autofill/content/renderer/password_autofill_agent.h
index 76f9fa0979cc4a8956d3d59e2f392d7adbd75b82..6d6900d7d7dfca025265a49760eba2a3fef77f95 100644
--- a/components/autofill/content/renderer/password_autofill_agent.h
+++ b/components/autofill/content/renderer/password_autofill_agent.h
@@ -147,6 +147,8 @@ class PasswordAutofillAgent : public content::RenderFrameObserver,
       const blink::WebSecurityOrigin& origin);
 
  private:
+  class FormElementObserver;
+
   // Ways to restrict which passwords are saved in ProvisionallySavePassword.
   enum ProvisionallySaveRestriction {
     RESTRICTION_NONE,
@@ -308,6 +310,8 @@ class PasswordAutofillAgent : public content::RenderFrameObserver,
 
   mojo::Binding<mojom::PasswordAutofillAgent> binding_;
 
+  FormElementObserver* form_element_observer_;  // Weak reference.
+
   DISALLOW_COPY_AND_ASSIGN(PasswordAutofillAgent);
 };