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.h

@@ -225,6 +225,9 @@ class AlloyBrowserHostImpl : public CefBrowserHostBase,
   bool TakeFocus(content::WebContents* source, bool reverse) override;
   bool HandleContextMenu(content::RenderFrameHost& render_frame_host,
                          const content::ContextMenuParams& params) override;
+  void CanDownload(const GURL& url,
+                   const std::string& request_method,
+                   base::OnceCallback<void(bool)> callback) override;
   content::KeyboardEventProcessingResult PreHandleKeyboardEvent(
       content::WebContents* source,
       const content::NativeWebKeyboardEvent& event) override;