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

third_party/WebKit/Source/core/dom/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//third_party/WebKit/Source/core/core.gni")
 
 blink_core_sources("dom") {
   split_count = 5
 
   sources = [
@@ skipping 138 matching lines @@
     "ElementVisibilityObserver.h",
     "EmptyNodeList.cpp",
     "EmptyNodeList.h",
     "ExceptionCode.h",
     "ExecutionContext.cpp",
     "ExecutionContext.h",
     "ExecutionContextTask.h",
     "FirstLetterPseudoElement.cpp",
     "FirstLetterPseudoElement.h",
     "FlexibleArrayBufferView.h",
+    "FormElementObserver.cpp",
+    "FormElementObserver.h",
     "FrameRequestCallback.h",
     "FrameRequestCallbackCollection.cpp",
     "FrameRequestCallbackCollection.h",
     "Fullscreen.cpp",
     "Fullscreen.h",
     "GlobalEventHandlers.h",
     "IconURL.cpp",
     "IconURL.h",
     "IdTargetObserver.cpp",
     "IdTargetObserver.h",
@@ skipping 277 matching lines @@
     "shadow/SlotAssignment.h",
     "shadow/SlotScopedTraversal.cpp",
     "shadow/SlotScopedTraversal.h",
   ]
 
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 }