Call close on the It2Me continue window

This fixes an issue reported on recent MacOS version, where the
ContinueWindow was not hidden after pressing either of the
buttons. This is the same issue that affected the Share
confirmation window, which was fixed in
http://crrev.com/c3ed375625dabcfe3ceaf2ca17146a6b10fc3bf1

Change-Id: Id2a3e913a3e9fa5c47d05fbbc2e2b314eae3612d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1785997
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Commit-Queue: Jamie Walch <jamiewalch@chromium.org>
Auto-Submit: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693409}

remoting/host/continue_window_mac.mm

@@ -152,7 +152,10 @@ - (void)hide {
     [window close];
   }
   shades_.reset();
-  continue_alert_.reset();
+  if (continue_alert_) {
+    [[continue_alert_ window] close];
+    continue_alert_.reset();
+  }
 }
 
 - (void)onCancel:(id)sender {