Implement TabDragDelegate for SxS entrypoints

This CL updates MultiContentsViewDropTargetController to implement
TabDragDelegate, allowing it to handle tab-dragging events.

This won't have any visible effects until a later CL, which will
implement Tab-dropping behaviour and connects this to the tab-dragging
system.

Change-Id: I50c9296bb2488659429fbbb15979e4226255627d
Bug: 394370034
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6719675
Reviewed-by: Shibalik Mohapatra <shibalik@chromium.org>
Reviewed-by: Darryl James <dljames@chromium.org>
Commit-Queue: Kaan Alsan <alsan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1487130}