Reenable MDCCollectionViewCellChrome.PreferredHeight

Bug: 776593
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I41d610437bbc4dda655b988d95c0491ea930871d
Reviewed-on: https://chromium-review.googlesource.com/809707
Commit-Queue: Louis Romero <lpromero@chromium.org>
Reviewed-by: Mike Baxley <baxley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522458}

ios/chrome/browser/ui/collection_view/cells/MDCCollectionViewCell+Chrome_unittest.mm

@@ -46,8 +46,7 @@ TEST_F(MDCCollectionViewCellChrome, PreferredHeightCallsConfigureCell) {
   EXPECT_EQ(1, item.configureCount);
 }
 
-// TODO(crbug.com/776593): Enable this test.
-TEST_F(MDCCollectionViewCellChrome, FLAKY_PreferredHeight) {
+TEST_F(MDCCollectionViewCellChrome, PreferredHeight) {
   CollectionViewFooterItem* footerItem =
       [[CollectionViewFooterItem alloc] initWithType:0];
   footerItem.text = @"This is a pretty lengthy sentence.";