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.cc

@@ -515,6 +515,21 @@ bool ClientHandler::OnCursorChange(CefRefPtr<CefBrowser> browser,
   return mouse_cursor_change_disabled_;
 }
 
+bool ClientHandler::CanDownload(CefRefPtr<CefBrowser> browser,
+                                const CefString& url,
+                                const CefString& request_method) {
+  CEF_REQUIRE_UI_THREAD();
+
+  if (!with_controls_) {
+    // Block the download.
+    LOG(INFO) << "Blocking download";
+    return false;
+  }
+
+  // Allow the download.
+  return true;
+}
+
 void ClientHandler::OnBeforeDownload(
     CefRefPtr<CefBrowser> browser,
     CefRefPtr<CefDownloadItem> download_item,