message: doc: add comment to clarify null param decision
diff: @@ -15,6 +15,9 @@ export class ContentScriptInjector {
  public injectScripts(tabId: number): Promise<void> {
  const inject = new Promise<null>(resolve => {
  ContentScriptInjector.cssFiles.forEach(file => {
  + // we need js to be injected in all frames before we start sending message, but,
  + // we do not need to wait for css, since css loading doesn't affect our messaging,
  + // hence the null callback here.
  this.chromeAdapter.injectCss(tabId, file, null);
  });
  