Bug 1720570 - Add CreateCssModule; r=allstarschh

This adds a method to create a css module to the modules interface.
Unlike JavaScript or JSON modules, we don't have an ability to parse
CSS inside SpiderMonkey, so this method accepts an already parsed
CSSStyleSheet value from the caller, and wraps it in a module.

Differential Revision: https://phabricator.services.mozilla.com/D263410

Author: dminor

js/public/Modules.h

@@ -194,6 +194,15 @@ extern JS_PUBLIC_API JSObject* CompileJsonModule(
     JSContext* cx, const ReadOnlyCompileOptions& options,
     SourceText<mozilla::Utf8Unit>& srcBuf);
 
+/**
+ * Create a synthetic module record for a CSS module from the provided
+ * CSSStyleSheet in cssValue. There's no capability to parse CSS in
+ * the engine, so this must occur prior to calling this function.
+ */
+extern JS_PUBLIC_API JSObject* CreateCssModule(
+    JSContext* cx, const ReadOnlyCompileOptions& options,
+    const Value& cssValue);
+
 /**
  * Set a private value associated with a source text module record.
  */

js/src/vm/Modules.cpp

@@ -294,6 +294,36 @@ JS_PUBLIC_API JSObject* JS::CompileJsonModule(
   return moduleObject;
 }
 
+JS_PUBLIC_API JSObject* JS::CreateCssModule(
+    JSContext* cx, const ReadOnlyCompileOptions& options,
+    const Value& cssValue) {
+
+  Rooted<ExportNameVector> exportNames(cx);
+  if (!exportNames.append(cx->names().default_)) {
+    ReportOutOfMemory(cx);
+    return nullptr;
+  }
+
+  Rooted<ModuleObject*> moduleObject(
+      cx, ModuleObject::createSynthetic(cx, &exportNames));
+  if (!moduleObject) {
+    return nullptr;
+  }
+
+  RootedVector<Value> exportValues(cx);
+  if (!exportValues.append(cssValue)) {
+    ReportOutOfMemory(cx);
+    return nullptr;
+  }
+
+  if (!ModuleObject::createSyntheticEnvironment(cx, moduleObject,
+                                                exportValues)) {
+    return nullptr;
+  }
+
+  return moduleObject;
+}
+
 JS_PUBLIC_API void JS::SetModulePrivate(JSObject* module, const Value& value) {
   JSRuntime* rt = module->zone()->runtimeFromMainThread();
   module->as<ModuleObject>().scriptSourceObject()->setPrivate(rt, value);