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### Issue Details

* **Electron Version:**
  * 8.2.2
* **Operating System:**
  * macOS 10.15.4
* **Last Known Working Electron version:**
  * N/A

### Expected Behavior
[before-input-event](https://github.com/electron/electron/blob/master/docs/api/web-contents.md#event-before-input-event) include the `isComposing` property of the [KeyboardEvents](https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent) object.

### Actual Behavior
There is no `KeyboardEvent.isComposing` property.

### To Reproduce

```javascript
webContents.on('before-input-event', (event, input) => {
  console.log('Is composing defined?', input.isComposing !== undefined);
});
```

### Screenshots
Here is the shape of an event is currently fired.
![image](https://user-images.githubusercontent.com/4505008/80048100-12145400-84c4-11ea-9ae9-7115fdc22c55.png)


### Additional Information
<!-- Add any other context about the problem here. -->

shell/common/gin_converters/content_converter.cc
#include "shell/common/gin_converters/content_converter.h"
#include <string>
#include <vector>
#include "content/public/browser/context_menu_params.h"
#include "content/public/browser/native_web_keyboard_event.h"
#include "content/public/browser/web_contents.h"
#include "content/public/common/untrustworthy_context_menu_params.h"
#include "shell/browser/api/electron_api_web_contents.h"
#include "shell/browser/web_contents_permission_helper.h"
#include "shell/common/gin_converters/blink_converter.h"
#include "shell/common/gin_converters/callback_converter.h"
#include "shell/common/gin_converters/gurl_converter.h"
#include "shell/common/gin_helper/dictionary.h"
#include "ui/events/keycodes/dom/keycode_converter.h"
#include "ui/events/keycodes/keyboard_code_conversion.h"
namespace {
void ExecuteCommand(content::WebContents* web_contents,
                    int action,
                    const content::CustomContextMenuContext& context) {
  web_contents->ExecuteCustomContextMenuCommand(action, context);
}
v8::Local<v8::Value> MenuToV8(v8::Isolate* isolate,
                              content::WebContents* web_contents,
                              const content::CustomContextMenuContext& context,
                              const std::vector<content::MenuItem>& menu);
v8::Local<v8::Value> MenuItemToV8(