[Delayed Clipboard Rendering] Privacy issue while reading data for web custom types

@annevk commented this on the webkit standard positions repo:

Since copying is not mediated by the user agent (other than requiring user activation) we have these concerns:

• Websites could use this to figure out what apps or websites the user is copying data towards.
• Websites could use this to figure out user habits. This is a lesser concern.

The first of these could perhaps be remedied by restricting the feature to the "builtin" clipboard types. That would not satisfy w3c/editing#417 (comment). We're not sure about a solution that does.

It also seems likely that the website the user copied from is not available for one reason or another. Websites can get OOM'd without notice. This relates to the user habits concern, but is also a practical concern as the feature might not be that useful on certain devices.

@evanstade @inexorabletash @sanketj We want to discuss possible mitigations for this issue.

Start with the privacy concerns in issue #439 and the referenced w3c/editing#417 discussion, then review the linked WebKit standard-position context. The issue does not name implementation files or tests; done means reaching agreement on a mitigation for identifying clipboard destinations and user habits.