**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()->Opener();
  if (frame && frame->IsWebLocalFrame())
    return frame->ToWebLocalFrame()->MainWorldScriptContext()->Global();
  else
    return v8::Null(isolate);
}
v8::Local<v8::Value> GetFrameParent(v8::Isolate* isolate,
                                    v8::Local<v8::Value> window) {
  blink::WebFrame* frame = GetRenderFrame(window)->GetWebFrame()->Parent();
  if (frame && frame->IsWebLocalFrame())
    return frame->ToWebLocalFrame()->MainWorldScriptContext()->Global();
  else
    return v8::Null(isolate);
}
v8::Local<v8::Value> GetTop(v8::Isolate* isolate, v8::Local<v8::Value> window) {
  blink::WebFrame* frame = GetRenderFrame(window)->GetWebFrame()->Top();
  if (frame && frame->IsWebLocalFrame())
    return frame->ToWebLocalFrame()->MainWorldScriptContext()->Global();
  else
    return v8::Null(isolate);
}
v8::Local<v8::Value> GetFirstChild(v8::Isolate* isolate,