**Is your feature request related to a problem? Please describe.**
After webContents.insertCSS() I cant find a way to remove injected stylesheet.

**Describe the solution you'd like**
insert: webContents.insertCSS('style-id', 'body { ... }') or .inserCSS('body { ... })'
remove: webContents.removeCSS('style-id') or .removeCSS() to remove all injected stylesheets


atom/renderer/api/atom_api_web_frame.cc
v8::Local<v8::Value> window) {
  blink::WebFrame* frame = GetRenderFrame(window)->GetWebFrame()->FirstChild();
  if (frame && frame->IsWebLocalFrame())
    return frame->ToWebLocalFrame()->MainWorldScriptContext()->Global();
  else
    return v8::Null(isolate);
}
v8::Local<v8::Value> GetNextSibling(v8::Isolate* isolate,
                                    v8::Local<v8::Value> window) {
  blink::WebFrame* frame = GetRenderFrame(window)->GetWebFrame()->NextSibling();
  if (frame && frame->IsWebLocalFrame())
    return frame->ToWebLocalFrame()->MainWorldScriptContext()->Global();
  else
    return v8::Null(isolate);
}
v8::Local<v8::Value> GetFrameForSelector(v8::Isolate* isolate,
                                         v8::Local<v8::Value> window,
                                         const std::string& selector) {
  blink::WebElement element =
      GetRenderFrame(window)->GetWebFrame()->GetDocument().QuerySelector(
          blink::WebString::FromUTF8(selector));
  if (element.IsNull())  
    return v8::Null(isolate);
  blink::WebFrame* frame = blink::WebFrame::FromFrameOwnerElement(element);
  if (frame && frame->IsWebLocalFrame())
    return frame->ToWebLocalFrame()->MainWorldScriptContext()->Global();
  else
    return v8::Null(isolate);
}
v8::Local<v8::Value> FindFrameByName(v8::Isolate* isolate,