**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
#include "atom/common/native_mate_converters/gfx_converter.h"
#include "atom/common/native_mate_converters/string16_converter.h"
#include "atom/common/node_includes.h"
#include "atom/common/promise_util.h"
#include "atom/renderer/api/atom_api_spell_check_client.h"
#include "base/memory/memory_pressure_listener.h"
#include "content/public/renderer/render_frame.h"
#include "content/public/renderer/render_frame_observer.h"
#include "content/public/renderer/render_frame_visitor.h"
#include "content/public/renderer/render_view.h"
#include "native_mate/dictionary.h"
#include "native_mate/object_template_builder.h"
#include "services/service_manager/public/cpp/interface_provider.h"
#include "third_party/blink/public/platform/web_cache.h"
#include "third_party/blink/public/platform/web_isolated_world_info.h"
#include "third_party/blink/public/web/web_custom_element.h"
#include "third_party/blink/public/web/web_document.h"
#include "third_party/blink/public/web/web_element.h"
#include "third_party/blink/public/web/web_frame_widget.h"
#include "third_party/blink/public/web/web_ime_text_span.h"
#include "third_party/blink/public/web/web_input_method_controller.h"
#include "third_party/blink/public/web/web_local_frame.h"
#include "third_party/blink/public/web/web_script_execution_callback.h"
#include "third_party/blink/public/web/web_script_source.h"
#include "third_party/blink/public/web/web_view.h"
#include "url/url_util.h"
namespace mate {
template <>
struct Converter<blink::WebLocalFrame::ScriptExecutionType> {
  static bool FromV8(v8::Isolate* isolate,