Fix ceftest failures with BackForwardCache enabled (see issue #2421)

When BackForwardCache is enabled the old RFH tree may be cached instead of being
immediately deleted as a result of main frame navigation. If a RFH is cached
then delivery of the CefFrameHandler::OnFrameDetached callback will be delayed
until the the RFH is ejected from the cache (possibly not occurring until the
browser is destroyed). This change in OnFrameDetached timing was causing
FrameHandlerTest.OrderSubCrossOrigin* to fail, and the inclusion of cached
frames in CefBrowserInfo::GetAllFrames was causing
FrameTest.NestedIframesDiffOrigin to fail.

BackForwardCache is currently being tested via field trials (see
https://crbug.com/1171298) and can be explicitly disabled using the
`--disable-back-forward-cache` or `--disable-features=BackForwardCache`
command-line flags.

libcef/browser/frame_host_impl.h

@@ -115,8 +115,9 @@ class CefFrameHostImpl : public CefFrame, public cef::mojom::BrowserFrame {
 
   // Owned frame objects will be detached explicitly when the associated
   // RenderFrame is deleted. Temporary frame objects will be detached
-  // implicitly via CefBrowserInfo::browser() returning nullptr.
-  void Detach();
+  // implicitly via CefBrowserInfo::browser() returning nullptr. Returns true
+  // if this was the first call to Detach() for the frame.
+  bool Detach();
 
   // cef::mojom::BrowserFrame methods forwarded from CefBrowserFrame.
   void SendMessage(const std::string& name, base::Value arguments) override;