[a11y] Enter the isolate before modifying v8 objects

Allocation and modifications of v8 objects require the embedder to have
entered the isolate.
This will be enforced more strictly with https://crrev.com/c/6387098.

This CL fixes tests and two places in production code where we did not
enter the isolate before manipulating objects.

R=nektar@chromium.org

Bug: 396607238
Change-Id: I382b7f8af8ed4ad4f7038cb53f943c8e398d9e88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6387048
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1437707}