[TabGroups] Add DialogMetadata struct to encapsulate dialog data

Adds the DialogMetadata struct which stores some of the previous
parameters to DeletionDialogController::MaybeShowDialog such as the
number of closing groups, if there are multiple grouped tab closures,
and the type of dialog.

This change allows us to flexibly introduce additional dialog types,
which can then add additional metadata to the struct. One example, is
the LeaveGroup type which will be implemented after this CL lands which
needs the title of the group being closed.

No behavioral changes.

Change-Id: I5f7a5237b0c434df282d048f02f314fe20f7978c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6096769
Reviewed-by: David Pennington <dpenning@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Darryl James <dljames@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1403844}