Also construct the `JSMap`s used to store these names lazily and
only on-demand, and construct them directly instead of first doing
a `std::vector<Handle<String>>` and using that to construct the
`JSMap`. The latter resulted in a gigantic root set of 150k+ handles,
which wasn't well received by the GC.

Bug: chromium:1154154
Fixed: chromium:1154564
Also-By: bmeurer@chromium.org
Change-Id: I92e8931f15eda133e2a62b5cc53fbe1f2dafcead
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2568275
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Auto-Submit: Philip Pfaffe <pfaffe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71589}