Make ModuleScript to have a reference to Modulator explicitly

This is to ensure the Modulator supplied when ModuleScript is created is also
used throughout its lifetime, especially when it is executed in
https://codereview.chromium.org/2819733002.

BUG=594639

Review-Url: https://codereview.chromium.org/2823483002
Cr-Commit-Position: refs/heads/master@{#464827}