Add runtime flag for <script type="module">

Also exercise it in a very basic way in ScriptLoader (at the moment,
<script type="module"> is treated the same as <script>).

See intent-to-implement:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/uba6pMr-jec/tXdg6YYPBAAJ

BUG=594639

Review URL: https://codereview.chromium.org/1807293004

Cr-Commit-Position: refs/heads/master@{#382083}
4 files changed