Use FIRST_PARENT constant in gr-change-view

Change-Id: I90a0c4efcbca8199c0cb5fb31733b5fad6be8a24
diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
index 53bdb91..9d65055 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
@@ -61,6 +61,7 @@
 import {RevisionInfo as RevisionInfoClass} from '../../shared/revision-info/revision-info';
 import {DiffViewMode} from '../../../api/diff';
 import {
+  DefaultBase,
   ChangeStatus,
   PrimaryTab,
   SecondaryTab,
@@ -1448,7 +1449,8 @@
     const parentCount = hasOwnProperty(parentCounts, 1) ? parentCounts[1] : 1;
 
     const preferFirst =
-      this._prefs && this._prefs.default_base_for_merges === 'FIRST_PARENT';
+      this._prefs &&
+      this._prefs.default_base_for_merges === DefaultBase.FIRST_PARENT;
 
     if (parentCount > 1 && preferFirst && !patchRange.patchNum) {
       return -1;