[ES6 modules] Fix bad assert in HostResolveImportedModule

Before this CL, ScriptModuleResolverImpl::Resolve wrongly assumed that
resolved module script's instantiation state is always "instantiated".
However, this isn't always the case when instantiating module graph w/
cycles.

This CL removes the CHECK.

Corresponding spec PR: https://github.com/whatwg/html/pull/2603

BUG=594639, https://github.com/whatwg/html/issues/2596
TEST=external/wpt/html/semantics/scripting-1/the-script-element/module/imports.html shouldn't crash

Review-Url: https://codereview.chromium.org/2848213002
Cr-Commit-Position: refs/heads/master@{#468555}
2 files changed