**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> val,
                     blink::WebLocalFrame::ScriptExecutionType* out) {
    std::string execution_type;
    if (!ConvertFromV8(isolate, val, &execution_type))
      return false;
    if (execution_type == "asynchronous") {
      *out = blink::WebLocalFrame::kAsynchronous;
    } else if (execution_type == "asynchronousBlockingOnload") {
      *out = blink::WebLocalFrame::kAsynchronousBlockingOnload;
    } else if (execution_type == "synchronous") {
      *out = blink::WebLocalFrame::kSynchronous;
    } else {
      return false;
    }
    return true;
  }
};
}  
namespace atom {
namespace api {
namespace {
content::RenderFrame* GetRenderFrame(v8::Local<v8::Value> value) {