Use placeholder in change list size col

Specifically. uses the placeholder for unknown sizes in the change list.

Bug: Issue 8467
Change-Id: I6d1ae5c3f5eaa18906832a380313ed577ff87262

polygerrit-ui/app/elements/change-list/gr-change-list-item/gr-change-list-item.html

@@ -210,7 +210,12 @@ limitations under the License.
       <gr-tooltip-content
           has-tooltip
           title="[[_computeSizeTooltip(change)]]">
-        <span>[[_computeChangeSize(change)]]</span>
+        <template is="dom-if" if="[[_changeSize]]">
+            <span>[[_changeSize]]</span>
+        </template>
+        <template is="dom-if" if="[[!_changeSize]]">
+            <span class="placeholder">--</span>
+        </template>
       </gr-tooltip-content>
     </td>
     <template is="dom-repeat" items="[[labelNames]]" as="labelName">

polygerrit-ui/app/elements/change-list/gr-change-list-item/gr-change-list-item.js

@@ -45,6 +45,10 @@
         value: false,
       },
       showNumber: Boolean,
+      _changeSize: {
+        type: String,
+        computed: '_computeChangeSize(change)',
+      },
     },
 
     behaviors: [
@@ -153,7 +157,7 @@
     _computeChangeSize(change) {
       const delta = change.insertions + change.deletions;
       if (isNaN(delta) || delta === 0) {
-        return '🤷'; // Unknown
+        return null; // Unknown
       }
       if (delta < CHANGE_SIZE.XS) {
         return 'XS';

polygerrit-ui/app/elements/change-list/gr-change-list-item/gr-change-list-item_test.html

@@ -226,7 +226,7 @@ limitations under the License.
       assert.equal(element._computeChangeSize({
         insertions: 'foo',
         deletions: 'bar',
-      }), '🤷');
+      }), null);
       assert.equal(element._computeChangeSize({
         insertions: 1,
         deletions: 1,