Get rid of usage of private _getElement from automerger.

Change-Id: Ibe463e7593ee971844bc5cb4486b26e99d6ef3ad
diff --git a/web/automerger.ts b/web/automerger.ts
index 96bcab5..cb0f499 100644
--- a/web/automerger.ts
+++ b/web/automerger.ts
@@ -32,11 +32,6 @@
   __url?: string;
 }
 
-export interface PopupPluginApiExtended extends PopupPluginApi {
-  // TODO: Remove this reference to a private method. This can break any time.
-  _getElement: () => HTMLElement;
-}
-
 export class Automerger {
   private change?: ChangeInfo;
 
@@ -94,9 +89,7 @@
     popupContent.appendChild(this.createMergeButton(branchToCheckbox));
 
     this.plugin.popup().then((popApi: PopupPluginApi) => {
-      const popupEl = (popApi as PopupPluginApiExtended)._getElement();
-      if (!popupEl) throw new Error('Popup element not found');
-      popupEl.appendChild(popupContent);
+      popup.appendContent(popupContent);
     });
   }