Refactor ui::Clipboard::ObjectMap sanitization in ClipboardMsgFilter.

      BUG=352395
R=tony@chromium.org
TBR=creis@chromium.org

Review URL: https://codereview.chromium.org/200523004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257164 0039d316-1c4b-4281-b951-d872f2087c98