Most of the users of these api methods manually ensure that the returned
values are Strings. With an additional flag we can easily ensure that already
in V8 and avoid needless api roundtrips.

Bug: v8:7358
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I62165d44084abc9f07f5bdaace5105847edca60a
Reviewed-on: https://chromium-review.googlesource.com/901248Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Franziska Hinkelmann <franzih@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51250}