[Merge to ToT] Fix side-panel crash.

The second CL referenced in the original CL has been reverted. As a
result, we are now merging the fix from M129 back to ToT.

> [M129/6668] Fix side-panel crash.
>
> This is an unusual CL that commits directly to 6668 without going
> through ToT. Please review carefully.
>
> commit 2fd6b53c introduces a crash that
> triggers when a tab showing a side-panel is discarded, and then later
> the side-panel is invoked again on that tab. Unfortunately, there are
> too many commits that depend on this commit to safely revert from M129.
>
> commit 00d39277 fixes this crash on
> ToT/M130 by changing the semantics of tab discard. Unfortunately, it is
> not safe to merge to M129.
>
> This CL is a M129-specific fix for the crash, as the crash is no longer
> present on ToT, and cannot be tested on ToT.
>
> This CL deregisters the about-site and extensions side-panel entries
> during tab-discard. These are the only two side-panels for which we've
> observed crashes.
>
> I manually tested by building is_chrome_branded and running through the
> following steps:
> (1) invoke tab-scoped side panel for feature <X>
> (2) switch to a new tab. switch back to confirm the side-panel still shows.
> (3) switch to chrome://discards and discard the tab.
> (4) switch to the original tab and confirm that either:
>   (4a) the side panel correctly shows.
>   (4b) if the side panel is hidden, showing it again does not cause a
>   crash.
>
> I tested with features: customize chrome, about this, extensions, lens
> overlay, read anything.
>
> Bug: 358851807
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5814932


> Commit-Queue: Erik Chen <erikchen@chromium.org>
> Reviewed-by: Eshwar Stalin <estalin@chromium.org>
> Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
> Cr-Commit-Position: refs/branch-heads/6668@{#406}
> Cr-Branched-From: 05bc6649-refs/heads/main@{#1343869}

Change-Id: Ib1679a3007d417cbf79f412afa98b1123aa736fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5828461


Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1349432}