Add ScopedTaskEnvironment in extensions::ModuleSystemTest.

This is a prerequisite to migrate v8_platform.cc from
base::WorkerPool to base::TaskScheduler.

BUG=708584
TBR=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2876123003
Cr-Commit-Position: refs/heads/master@{#471286}
2 files changed