This is in preparation to hold on to unregister tokens weakly. The key
map will be changed to be keyed off the tokens' identity hash instead of
the token objects themselves. Once changed, a WeakCell's key (its
token's hash) will be different from its unregister token. In
particular, in case of collision, WeakCells with different unregister
tokens may have the same key.

Bug: v8:8179
Change-Id: Ifa18ace915265340db7f01431161a6e0425f2927
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1968958
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65462}