[ES6 Modules] Introduce ModuleScriptFetchRequest

This CL introduces ModuleScriptFetchRequest, a parameter object which
will be used to represent arguments of the spec "fetch a single module script"
algorithm:
https://html.spec.whatwg.org/#fetch-a-single-module-script

For a graphical mapping to the spec concept, please see:
https://docs.google.com/spreadsheets/d/1tofYrhZrTl605CHk95VVj_1HN7lOkQ3C7D-5NRful4s/edit#gid=0

BUG=594639

Review-Url: https://codereview.chromium.org/2660663002
Cr-Commit-Position: refs/heads/master@{#447620}
2 files changed