WebUI Toolbar: Restrict dragging across divider

This CL prevents dragging pinned items past the divider, and disables
dragging for popped-out items.

- Disables draggable attribute on pinned-toolbar-action when poppedOut is true.
- Container passes poppedOut to elements based on divider index.
- Caps insert index in onActionDragover to dividerIndex - 1 for pinned items.
- Adds tests to verify poppedOut property and draggable/dragstart behavior.

BUG=493171983

Change-Id: I644783b2dcd433fb798f99756dd9f8e370661e43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8204804
Commit-Queue: Russ Hamilton <behamilton@google.com>
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1676714}