Optimize v8 object lookup in a11y tests.

Improves lookup from O(n) in number of observed objects to O(1).
As a result, tests that loop over a large number of elements improve
from O(n^2) to O(n). This happened for several tests, e.g:

external/wpt/mathml/presentation-markup/operators/operator-dictionary-spacing-005.html

Change-Id: Id58a9821f5db44e1f532c24abcb14e376d9c5eed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4908627
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1204980}