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

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

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_;