Introduce ModuleScript::CreateInternal()

This is preparation for https://codereview.chromium.org/2850673002
to create a place to insert RegisterModuleScript() both for
Create() and CreateForTest().

This doesn't change the behavior.

BUG=594639, 715369

Review-Url: https://codereview.chromium.org/2844413003
Cr-Commit-Position: refs/heads/master@{#467891}
2 files changed