The method returns names for all available top-level scope variables
in giving context.

R=dgozman@chromium.org,jgruber@chromium.org

Bug: chromium:681333
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I2d0b600e1afbfef9087f53ea9c26abe1e112047c
Reviewed-on: https://chromium-review.googlesource.com/719409
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48618}