Async clipboard API used to have same capability as DataTransfer for clipobard
read operation. But after this patch, we intent to use the nsIClipboard service
directly in async clipboard, which also includes some changes on mime types support
for read operation. Now async clipboard read supports only built-in types, i.e.
text/plain, text/html and image/png. This also align with other browsers.
Depends on D150164
Differential Revision: https://phabricator.services.mozilla.com/D145379
In D145379, ItemEntry will be used in lambda capture, it is much safer to make
it a reference counted object to ensue the captured object lives long enough.
Depends on D152516
Differential Revision: https://phabricator.services.mozilla.com/D150164