We want to remove GetIsolate calls, but this particular one is safe
and necessary to avoid either storing the isolate in the parser/scopes,
or passing it through as a parameter throughout the parser.

Bug: v8:7786
Change-Id: I07765f5c20b2c9925bb0b980bc9fe850c91d7811
Reviewed-on: https://chromium-review.googlesource.com/1104685Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53822}