[ios, tabgrid] Match background of containerView with GroupSnapshotView

When drag and drop is enabled, a new containerView is created to hold
the contents of the TabGroupCell contents to perform shrink
transforms. It needs to have the same background color as the
GroupSnapshotView.

Also changes the UIDropOperation for drag and drops to
UIDropOperationMove so the tab doesn't have a green plus icon in the
top right.

Change-Id: I641b7756499fdcb7ab457f1651d048b9f336abd4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7180666
Reviewed-by: Louis Romero <lpromero@google.com>
Auto-Submit: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Louis Romero <lpromero@google.com>
Cr-Commit-Position: refs/heads/main@{#1548399}