Add permission callback for user-initated downloads (fixes issue #3183)

This change adds a CefDownloadHandler::CanDownload callback for optionally
blocking user-initiated downloads (e.g. alt + link click or link click that
returns a `Content-Disposition: attachment` response from the server).

To test:
- Run `ceftests --gtest_filter=DownloadTest.*`.
- Run `cefclient --hide-controls`. User-initiated downloads will be blocked.

tests/cefclient/browser/client_handler.h

@@ -170,6 +170,9 @@ class ClientHandler : public CefClient,
                       const CefCursorInfo& custom_cursor_info) override;
 
   // CefDownloadHandler methods
+  bool CanDownload(CefRefPtr<CefBrowser> browser,
+                   const CefString& url,
+                   const CefString& request_method) override;
   void OnBeforeDownload(CefRefPtr<CefBrowser> browser,
                         CefRefPtr<CefDownloadItem> download_item,
                         const CefString& suggested_name,