Expand content::ClipboardEndpoint paste type support

This CL is done in preparation of refactoring the caching logic in
ClipboardHostImpl and doesn't change any current behavior.

This CL does the following refactors:
- The `content::ClipboardEndpoint::text` field is now only used for
  text pastes. Other kinds of paste have new fields in the struct.
- The UTF8 <-> UTF16 logic bits are moved out of `ClipboardHostImpl`
  into enterprise code.
- Enterprise code is updated to correctly handle the refactor.

Bug: b/294844565, b/313091312
Change-Id: Ie61d6d5012c16f636074bf879747d639e1fd82b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5301597


Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Nancy Xiao <nancylanxiao@google.com>
Commit-Queue: Dominique Fauteux-Chapleau <domfc@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1261878}