Bug 1712122 - Part 3: Cancel pending write request when a new write request is made; r=nika,geckoview-reviewers,m_kato

The Async Clipboard API now allows using arbitrary promises for passing write data,
potentially enabling websites to delay writing data to an arbitrary future, which
may surprise the user. This patch introduces a solution: a new write request will
automatically cancel any previous pending request.

To implement that, this patch introduces a new method to nsIClipboard, new XPCOM
interfaces, and new IPC to efficiently track individual write requests. Additionally,
a new helper base class, ClipboardSetDataHelper, is introduced in widget to facilitate
platform code sharing.

Differential Revision: https://phabricator.services.mozilla.com/D174090