[Tab Grid Dialog] Cleanup and docs around BINDING_TOKEN

Added the requested comments as per previous CL:
https://chromium-review.googlesource.com/c/chromium/src/+/4684992

Having spent some additional time considering the INDIVIDUAL_KEYS
proposal it is likely to cause more headaches than fixes.
1) Future developers need to now know about INDIVIDUAL_KEYS and
   whether any new property complies. The same could be said about
   BINDING_TOKEN, although that behavior is less nuanced to grasp.
2) Very few properties can safely be INDIVIDUAL_KEYS. At present,
   most properties depend on state tracked in the Mediator:
   * ADD_BUTTON_CLICK_LISTENER for metrics.
   * COLLAPSE_BUTTON_CONTENT_DESCRIPTION for the stored title.
   * TEXT_WATCHER to track title correctly.
   It is surprising this hadn't already caused issues in the past.
   In fact, it is likely some of our UserActions have been
   miscounted as a result of this.

Admittedly this should be safe for shared things like colors, but it is
easier to reason about if the state of the UI is self consistent with
the active model rather than potentially having stale state.

It is true that developers just have to worry about null checks.
However, if something is actually broken by setting it to null the unit
test TabGridPanelViewBinderTest#testBindingToken() should catch the
issue.

Fixed: 1464665
Change-Id: I5e139cca541b64e4670fc1b00eb0a4011f10cacb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4706924
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Yue Zhang <yuezhanggg@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Mei Liang <meiliang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1173459}