**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,
                                     const std::string& name) {
  blink::WebFrame* frame =
      GetRenderFrame(window)->GetWebFrame()->FindFrameByName(
          blink::WebString::FromUTF8(name));
  if (frame && frame->IsWebLocalFrame())
    return frame->ToWebLocalFrame()->MainWorldScriptContext()->Global();
  else
    return v8::Null(isolate);
}
int GetRoutingId(v8::Local<v8::Value> window) {
  return GetRenderFrame(window)->GetRoutingID();
}
}  
}  
namespace {
void Initialize(v8::Local<v8::Object> exports,
                v8::Local<v8::Value> unused,
                v8::Local<v8::Context> context,
                void* priv) {
  using namespace atom::api;  
  v8::Isolate* isolate = context->GetIsolate();
  mate::Dictionary dict(isolate, exports);
  dict.SetMethod("setName", &SetName);
  dict.SetMethod("setZoomLevel", &SetZoomLevel);