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Ref #19447

### Issue Details

* **Electron Version:**
  * 9.0.0-beta.24
* **Operating System:**
  * macOS 10.15, Windows 10
* **Last Known Working Electron version:**
  * none

### Expected Behavior
`e.sender` object received from an extension background page should have `webPreferences` and `session` initialized. It looks like `options` in `api::WebContents` constructor don't get initialized properly for an extension background page.

### Actual Behavior
`e.sender` object doesn't have `session` and `e.sender.getWebPreferences()` returns `null`.

### To Reproduce
https://github.com/sentialx/electron-quick-start/tree/extensions/no-session

Just run it and you will see something like this:
![image](https://user-images.githubusercontent.com/11065386/82097607-82994400-9703-11ea-8291-0eb987d8aca0.png)

### Additional Information
This is a real blocker for anyone who would want to provide own `chrome` APIs in background pages initialized from different sessions.
shell/browser/api/electron_api_web_contents.cc
}
uint32_t Ws::FindInPage(gin_helper::Arguments* args) {
  base::string16 search_text;
  if (!args->GetNext(&search_text) || search_text.empty()) {
    args->ThrowError("Must provide a non-empty search content");
    return 0;
  }
  uint32_t request_id = GetNextRequestId();
  gin_helper::Dictionary dict;
  auto options = blink::mojom::FindOptions::New();
  if (args->GetNext(&dict)) {
    dict.Get("forward", &options->forward);
    dict.Get("matchCase", &options->match_case);
    dict.Get("findNext", &options->find_next);
  }
  ontents()->Find(request_id, search_text, std::move(options));
  return request_id;
}
void Ws::StopFindInPage(content::StopFindAction action) {
  ontents()->StopFinding(action);
}
void Ws::ShowDefinitionForSelection() {
#if defined(OS_MACOSX)
  auto* const view = ontents()->GetRenderWidgetHostView();
  if (view)
    view->ShowDefinitionForSelection();
#endif
}
void Ws::CopyImageAt(int x, int y) {
  auto* const host = ontents()->GetMainFrame();