[iOS][PT] Display the pinned view when dropping from another app

This CL displays the pinned view when a drop interaction comes from
another window/app.

To do that, when an a drop interaction is detected in the grid view,
it notifies its delegate which triggers `dragSessionEnabled:`.

demo:
https://drive.google.com/file/d/1qrlcJDaJ39Z7IrN4TUwb9rUqysug6KuA/view?usp=share_link&resourcekey=0-fU1uN2_uJAAvJlsV6qGIJw

Fixed: 1408732
Change-Id: Ibd8888d240f3201474563888bf5d5d53fda31705
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4196635
Commit-Queue: Ewann Pellé <ewannpv@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1097368}