**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
dict.SetMethod("getZoomLevel", &GetZoomLevel);
  dict.SetMethod("setZoomFactor", &SetZoomFactor);
  dict.SetMethod("getZoomFactor", &GetZoomFactor);
  dict.SetMethod("setVisualZoomLevelLimits", &SetVisualZoomLevelLimits);
  dict.SetMethod("setLayoutZoomLevelLimits", &SetLayoutZoomLevelLimits);
  dict.SetMethod("allowGuestViewElementDefinition",
                 &AllowGuestViewElementDefinition);
  dict.SetMethod("getWebFrameId", &GetWebFrameId);
  dict.SetMethod("setSpellCheckProvider", &SetSpellCheckProvider);
  dict.SetMethod("insertText", &InsertText);
  dict.SetMethod("insertCSS", &InsertCSS);
  dict.SetMethod("executeJavaScript", &ExecuteJavaScript);
  dict.SetMethod("executeJavaScriptInIsolatedWorld",
                 &ExecuteJavaScriptInIsolatedWorld);
  dict.SetMethod("setIsolatedWorldInfo", &SetIsolatedWorldInfo);
  dict.SetMethod("getResourceUsage", &GetResourceUsage);
  dict.SetMethod("clearCache", &ClearCache);
  dict.SetMethod("_findFrameByRoutingId", &FindFrameByRoutingId);
  dict.SetMethod("_getFrameForSelector", &GetFrameForSelector);
  dict.SetMethod("_findFrameByName", &FindFrameByName);
  dict.SetMethod("_getOpener", &GetOpener);
  dict.SetMethod("_getParent", &GetFrameParent);
  dict.SetMethod("_getTop", &GetTop);
  dict.SetMethod("_getFirstChild", &GetFirstChild);
  dict.SetMethod("_getNextSibling", &GetNextSibling);
  dict.SetMethod("_getRoutingId", &GetRoutingId);
}
}  
NODE_LINKED_MODULE_CONTEXT_AWARE(atom_renderer_web_frame, Initialize)