This folds the allocation of the JSRegExpResult (basically a JSArray)
together with its elements fixed array.

Bug: v8:6741
Change-Id: I027c4fce7169c1bd6a17637619fdf3890e3f7f24
Reviewed-on: https://chromium-review.googlesource.com/625877
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47536}