Add erase function to gr-storage

Functions were added to gr-storage for managing editable content, but
erase was forgotten.

Bug: Issue 6309
Change-Id: Ie44e13f8448ebabc394b5b7c6b6c76c5dc84889e
diff --git a/polygerrit-ui/app/elements/shared/gr-storage/gr-storage_test.html b/polygerrit-ui/app/elements/shared/gr-storage/gr-storage_test.html
index b1a77c7..68d92cb 100644
--- a/polygerrit-ui/app/elements/shared/gr-storage/gr-storage_test.html
+++ b/polygerrit-ui/app/elements/shared/gr-storage/gr-storage_test.html
@@ -184,6 +184,10 @@
       assert.equal(item.message, 'my content');
       assert.isOk(item.updated);
       assert.isTrue(cleanupStub.called);
+
+      // eraseEditableContentItem performs as expected.
+      element.eraseEditableContentItem(key);
+      assert.isNotOk(element._storage.getItem(key));
     });
   });
 </script>