[TabStripRefactor] Fixed crash with source view drag drop.

Bug caused due to CL: https://chromium-review.googlesource.com/c/chromium/src/+/6156355

Issue: When we drag and drop tab into another instance, clearTabDragState is called on drag end. This cleans up viewBeingDragged and lastOff but does not set mActiveStrategy to null. Then when you try to reorder again on source strip, startReorder fails with assertion since mActiveStrategy != null.

Includes additional minor cleanup.

Bug: 381285152
Change-Id: I2760eaf31925b0a26585a2d377528e51fe7a7561
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6161095
Commit-Queue: Sirisha Kavuluru <skavuluru@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Neil Coronado <nemco@google.com>
Reviewed-by: Zhe Li <zheliooo@google.com>
Cr-Commit-Position: refs/heads/main@{#1403948}