src/vs/workbench/api/browser/mainThreadWebviews.ts/MainThreadWebviews/addWebview
class MainThreadWebviews:	public addWebview(handle: extHostProtocol.WebviewHandle, webview: IOverlayWebview, options: { serializeBuffersForPostMessage: boolean }): void {
		if (this._webviews.has(handle)) {
			throw new Error('Webview already registered');
		}

		this._webviews.set(handle, webview);
		this.hookupWebviewEventDelegate(handle, webview, options);
	}