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### Issue Details

* **Electron Version:**
  * 10.1.3
* **Operating System:**
  * win7 sp1 x64

- without worldSafeExecuteJavaScript, `(webFrame|webContents).executeJavaScript('new Promise(resolve => setTimeout(() => resolve(2), 3000));')` works as expected; the executeJavaScript promise waits for the eval promise.
- in the case of webFrame the evaled promise is returned raw
- in the case of webContents the promise is marshaled
- with worldSafeExecuteJavaScript, both error with: "An object could not be cloned"
- webContents should continue marshaling promises, otherwise its an unnecessary breaking change
- webFrame should marshal promises for consistency with webContents
- the clone restriction would be on the promise resolve value

### To Reproduce
`(webFrame|webContents).executeJavaScript('new Promise(resolve => setTimeout(() => resolve(2), 3000));')`


shell/renderer/api/electron_api_web_frame.cc
const std::string& css,
                         gin_helper::Arguments* args) {
  blink::WebDocument::CSSOrigin css_origin =
      blink::WebDocument::CSSOrigin::kAuthorOrigin;
  gin_helper::Dictionary options;
  if (args->GetNext(&options))
    options.Get("cssOrigin", &css_origin);
  auto* render_frame = GetRenderFrame(window);
  if (!render_frame) {
    args->ThrowError(
        "Render frame was torn down before webFrame.insertCSS could be "
        "executed");
    return base::string16();
  }
  blink::WebFrame* web_frame = render_frame->GetWebFrame();
  if (web_frame->IsWebLocalFrame()) {
    return web_frame->ToWebLocalFrame()
        ->GetDocument()
        .InsertStyleSheet(blink::WebString::FromUTF8(css), nullptr, css_origin)
        .Utf16();
  }
  return base::string16();
}
void RemoveInsertedCSS(gin_helper::ErrorThrower thrower,