Add CHECKs to try to narrow down cause of bad internal fields in Window DOM wrapper

third_party/WebKit/Source/web/WebRemoteFrameImpl.cpp

Index: third_party/WebKit/Source/web/WebRemoteFrameImpl.cpp
diff --git a/third_party/WebKit/Source/web/WebRemoteFrameImpl.cpp b/third_party/WebKit/Source/web/WebRemoteFrameImpl.cpp
index 3fcba5514d4016c656052ed1c962fa0d90d7a9cf..fc2e9215b284b67b322f5a16ff27f740c565b5c6 100644
--- a/third_party/WebKit/Source/web/WebRemoteFrameImpl.cpp
+++ b/third_party/WebKit/Source/web/WebRemoteFrameImpl.cpp
@@ -5,6 +5,7 @@
 #include "web/WebRemoteFrameImpl.h"
 
 #include "bindings/core/v8/RemoteWindowProxy.h"
+#include "bindings/core/v8/V8Window.h"
 #include "core/dom/Fullscreen.h"
 #include "core/dom/RemoteSecurityContext.h"
 #include "core/dom/SecurityContext.h"
@@ -26,6 +27,7 @@
 #include "web/RemoteFrameOwner.h"
 #include "web/WebLocalFrameImpl.h"
 #include "web/WebViewImpl.h"
+#include "wtf/debug/Alias.h"
 
 namespace blink {
 
@@ -215,9 +217,23 @@ v8::Local<v8::Context> WebRemoteFrameImpl::mainWorldScriptContext() const {
 
 v8::Local<v8::Context> WebRemoteFrameImpl::deprecatedMainWorldScriptContext()
     const {
-  return static_cast<RemoteWindowProxy*>(
-             frame()->windowProxy(DOMWrapperWorld::mainWorld()))
-      ->contextIfInitialized();
+  v8::Local<v8::Context> context =
+      static_cast<RemoteWindowProxy*>(
+          frame()->windowProxy(DOMWrapperWorld::mainWorld()))
+          ->contextIfInitialized();
+
+  DOMWindow* window = frame()->domWindow();
+  // Prevent this local from getting optimized out, and hopefully, the heap
+  // contents captured into minidumps.
+  WTF::debug::alias(&window);
+
+  v8::Local<v8::Object> globalProxy = context->Global();
+  CHECK(!globalProxy.IsEmpty());
+  CHECK(V8Window::hasInstance(globalProxy, v8::Isolate::GetCurrent()));
+  CHECK(window);
+  CHECK_EQ(window, V8Window::toImpl(globalProxy));
+
+  return context;
 }
 
 void WebRemoteFrameImpl::reload(WebFrameLoadType) {