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

polygerrit-ui/app/elements/shared/gr-storage/gr-storage.js

@@ -58,14 +58,18 @@
       this._storage.removeItem(key);
     },
 
+    getEditableContentItem(key) {
+      this._cleanupItems();
+      return this._getObject(this._getEditableContentKey(key));
+    },
+
     setEditableContentItem(key, message) {
       this._setObject(this._getEditableContentKey(key),
           {message, updated: Date.now()});
     },
 
-    getEditableContentItem(key) {
-      this._cleanupItems();
-      return this._getObject(this._getEditableContentKey(key));
+    eraseEditableContentItem(key) {
+      this._storage.removeItem(key);
     },
 
     getPreferences() {

polygerrit-ui/app/elements/shared/gr-storage/gr-storage_test.html

@@ -184,6 +184,10 @@ limitations under the License.
       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>