Perf - Fix alert-page-sk. We never saw diffs.

Bug: skia:9219
Change-Id: I15e10c0fadbbb63160b3a0663cfe3ff30962c8fb
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/234056
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
diff --git a/perf/modules/alerts-page-sk/alerts-page-sk.js b/perf/modules/alerts-page-sk/alerts-page-sk.js
index 1f10a33..25cf034 100644
--- a/perf/modules/alerts-page-sk/alerts-page-sk.js
+++ b/perf/modules/alerts-page-sk/alerts-page-sk.js
@@ -134,8 +134,7 @@
     const id = +window.location.search.slice(1);
     for (const alert of this._alerts) {
       if (id === alert.id) {
-        this.cfg = alerts[i];
-        this._dialog.showModal();
+        this._startEditing(alerts[i]);
         break;
       }
     }
@@ -145,14 +144,17 @@
   _add() {
     // Load an new Config from the server.
     fetch('/_/alert/new').then(jsonOrThrow).then((json) => {
-      this.cfg = json;
-      // Pop up edit dialog using the new Config.
-      this._dialog.showModal();
+      this._startEditing(json);
     }).catch(errorMessage);
   }
 
   _edit(e) {
-    this.cfg = e.target.__config;
+    this._startEditing(e.target.__config);
+  }
+
+  _startEditing(cfg) {
+    this._orig_cfg = JSON.parse(JSON.stringify(this._cfg));
+    this.cfg = cfg;
     this._dialog.showModal();
   }
 
@@ -196,7 +198,6 @@
     if (this._cfg && !this._cfg.owner) {
       this._cfg.owner = this._email;
     }
-    this._orig_cfg = JSON.parse(JSON.stringify(this._cfg));
     this._render();
   }