Change HandleOnPerformDrop signature to return DropData as a result

This change is necessary to fix b/243569406 as the drop data might get
its files modified, so instead of returning a block/allow enum value
the callback now returns an optional DropData object. This doesn't
change the current implementation functionnality-wise and just
updates tests and callers of HandleOnPerformDrop.

Bug: b/243569406
Change-Id: Ic7b2d13b96b39b7631a73905db4fc98b786c04a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3883838
Commit-Queue: Dominique Fauteux-Chapleau <domfc@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1048752}