This is always equal to isolate_ (so getting it from the isolate is totally
free), and this makes the code slightly smaller while avoiding a few loads
and stores here and there.

Change-Id: Ib648cd0a1f6ea7cbb73a0188ec92cd091ce34792
Reviewed-on: https://chromium-review.googlesource.com/794046Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49758}