[MagicStack] Expose tracking Tab from ModuleDelegate.

In this CL, we add ModuleDelegate#getTrackingTab() API. It allows to
get the local Tab which is showing on the magic stack via
ModuleDelegate.

Bug: 332588018
Change-Id: Iac7d00dcdc83e1456877d3335202c3a2b479e202
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5420095


Reviewed-by: Xinyi Ji <xinyiji@chromium.org>
Commit-Queue: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281883}