exo: ShellClientData: Use Wayland destroy() functions.

std::unique_ptr's default Deleter calls `delete`. But Wayland proxy
objects are allocated using `malloc` (actually `calloc`) and therefore
should be deallocated using the appropriate *_destroy() functions, which
internally call `free`.

Therefore, whenever a std::unique_ptr references a Wayland proxy object,
we must supply the appropriate destroy function as a custom Deleter.

BUG=none, but see discussion at https://crrev.com/c/5481467

Change-Id: I5a905edc2caad2cb46b20c6713ec45ebe6377402
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5514087
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Auto-Submit: Chloe Pelling <cpelling@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1298128}