multipaste: Make ClipboardHistoryItems notify observers of image changes

This CL introduces the ability to register a callback with a
ClipboardHistoryItem to run when the item's HTML preview finishes
rendering. This takes the place of observing the
ClipboardHistoryResourceManager, allowing the manager to be plumbed
through fewer places in the code.

Fixed: b/279093872
Change-Id: Ia5c346e4ac4fac60f674be981eee73b79fd3ed32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4471678
Reviewed-by: David Black <dmblack@google.com>
Commit-Queue: Colin Kincaid <ckincaid@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1136260}