[gin] Remove assert requiring two embedder fields for ABs

This assert made sure that V8 was compiled with two embedder fields
for ABs to be able to dynamically use them as wrappers for
DOMArrayBuffer without creating an intermediate JS API object.

ABs now have a dedicated field for CppHeapPointers for this
purpose.

Bug: 346350847
Change-Id: I2552f90fcdc59a3046a0beea34aafd17bbf6a859
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5621930


Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313445}