Support Inline module script

This CL
- Enables inline module scripts in ScriptLoader.
- Adds ModuleTreeLinker::FetchDescendantsForInlineScript() and
  related plumbing to ModuleTreeLinkerRegistry and Modulator.
  This is not yet spec'ed and we might need revisit once the spec is
  updated. https://github.com/whatwg/html/issues/2544
- Makes ModuleScript to store its |source_text| for CSP of inline scripts.

BUG=715369, 594639

Review-Url: https://codereview.chromium.org/2842923002
Cr-Commit-Position: refs/heads/master@{#468087}
13 files changed