alloy: Fix FrameHandlerTest failures with BackForwardCache enabled (see issue #2421)

To test:
Run `ceftests --gtest_filter=FrameHandlerTest.OrderSub*
              --enable-features=BackForwardCache`

libcef/browser/browser_contents_delegate.cc

@@ -365,6 +365,10 @@ void CefBrowserContentsDelegate::DidFinishNavigation(
       (error_code == net::OK ? navigation_handle->GetURL() : GURL());
 
   auto browser_info = browser_info_;
+  if (!browser_info->browser()) {
+    // Ignore notifications when the browser is closing.
+    return;
+  }
 
   // May return NULL when starting a new navigation if the previous navigation
   // caused the renderer process to crash during load.

libcef/browser/browser_host_base.cc

@@ -798,6 +798,7 @@ void CefBrowserHostBase::OnBeforeClose() {
       handler->OnBeforeClose(this);
     }
   }
+  browser_info_->SetClosing();
 }
 
 void CefBrowserHostBase::OnBrowserDestroyed() {

libcef/browser/browser_info.cc

@@ -41,7 +41,9 @@ CefBrowserInfo::~CefBrowserInfo() {
 
 CefRefPtr<CefBrowserHostBase> CefBrowserInfo::browser() const {
   base::AutoLock lock_scope(lock_);
-  return browser_;
+  if (!is_closing_)
+    return browser_;
+  return nullptr;
 }
 
 void CefBrowserInfo::SetBrowser(CefRefPtr<CefBrowserHostBase> browser) {
@@ -71,6 +73,12 @@ void CefBrowserInfo::SetBrowser(CefRefPtr<CefBrowserHostBase> browser) {
   }
 }
 
+void CefBrowserInfo::SetClosing() {
+  base::AutoLock lock_scope(lock_);
+  DCHECK(!is_closing_);
+  is_closing_ = true;
+}
+
 void CefBrowserInfo::MaybeCreateFrame(content::RenderFrameHost* host,
                                       bool is_guest_view) {
   CEF_REQUIRE_UIT();
@@ -228,7 +236,7 @@ void CefBrowserInfo::RemoveFrame(content::RenderFrameHost* host) {
 CefRefPtr<CefFrameHostImpl> CefBrowserInfo::GetMainFrame() {
   NotificationStateLock lock_scope(this);
   // Early exit if called post-destruction.
-  if (!browser_)
+  if (!browser_ || is_closing_)
     return nullptr;
 
   CHECK(main_frame_);

libcef/browser/browser_info.h

@@ -52,6 +52,13 @@ class CefBrowserInfo : public base::RefCountedThreadSafe<CefBrowserInfo> {
   // (to set) and DestroyBrowser (to clear).
   void SetBrowser(CefRefPtr<CefBrowserHostBase> browser);
 
+  // Called after OnBeforeClose and before SetBrowser(nullptr). This will cause
+  // browser() and GetMainFrame() to return nullptr as expected by
+  // CefFrameHandler callbacks. Note that this differs from calling
+  // SetBrowser(nullptr) because the WebContents has not yet been destroyed and
+  // further frame-related callbacks are expected.
+  void SetClosing();
+
   // Ensure that a frame record exists for |host|. Called for the main frame
   // when the RenderView is created, or for a sub-frame when the associated
   // RenderFrame is created in the renderer process.
@@ -239,6 +246,9 @@ class CefBrowserInfo : public base::RefCountedThreadSafe<CefBrowserInfo> {
   // The current main frame.
   CefRefPtr<CefFrameHostImpl> main_frame_;
 
+  // True if the browser is currently closing.
+  bool is_closing_ = false;
+
   // Only accessed on the UI thread.
   std::vector<CefDraggableRegion> draggable_regions_;