Use the Isolate* version of Concat.

Bug: v8:7754
Change-Id: I3d16405032ab0690c57e2ba615cac60d8fa92464
Reviewed-on: https://chromium-review.googlesource.com/1146578Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54608}