alloy: Enable BackForwardCache support (see issue #2421)

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. The default behavior now matches the Chrome runtime.

libcef/browser/alloy/alloy_browser_host_impl.h

@@ -289,6 +289,8 @@ class AlloyBrowserHostImpl : public CefBrowserHostBase,
       const viz::SurfaceId& surface_id,
       const gfx::Size& natural_size) override;
   void ExitPictureInPicture() override;
+  bool IsBackForwardCacheSupported() override;
+  bool IsPrerender2Supported() override;
 
   // content::WebContentsObserver methods.
   using content::WebContentsObserver::BeforeUnloadFired;