Move "create a module script" from ModuleScriptLoader to ModuleScript

Because it will be used from outside ModuleScriptLoader when we support
inline scripts.

This CL
- Renames existing ModuleScript::Create() (which is a simple wrapper of
  the constructor) to CreateForTest(),
- Moves ModuleScriptLoader::CreateModuleScript() to new
  ModuleScript::Create(), and
- Reorders the parameters to match with the spec.

BUG=594639, 715369

Review-Url: https://codereview.chromium.org/2845743003
Cr-Commit-Position: refs/heads/master@{#467873}
7 files changed