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.

libcef/browser/alloy/alloy_browser_host_impl.cc

@@ -1225,6 +1225,13 @@ bool AlloyBrowserHostImpl::HandleContextMenu(
   return HandleContextMenu(web_contents(), params);
 }
 
+void AlloyBrowserHostImpl::CanDownload(
+    const GURL& url,
+    const std::string& request_method,
+    base::OnceCallback<void(bool)> callback) {
+  contents_delegate_->CanDownload(url, request_method, std::move(callback));
+}
+
 KeyboardEventProcessingResult AlloyBrowserHostImpl::PreHandleKeyboardEvent(
     content::WebContents* source,
     const content::NativeWebKeyboardEvent& event) {