[js weak refs] Fix jumbo build

The test defined a function called "Register" which clashes with other functions
with the same name.

NOTRY=true
TBR=sigurds@chromium.org
BUG=v8:8179

Change-Id: I7ddfc481cd1de750272de7b43e2e7f9fe26626a9
Reviewed-on: https://chromium-review.googlesource.com/c/1445982Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59208}