DisplayItemBuilder now has methods:
```
void StartGroup(nsPaintedDisplayItem* aItem);
void CancelGroup();
void FinishGroup();
bool ReuseItem(nsPaintedDisplayItem* aItem);
```
WebRender display items previously created between calls to StartGroup() and FinishGroup() will be reused by a call to ReuseItem(),
which will push DisplayItem::ReuseItem(key) to WR display list, if the Gecko display item has been retained and reused.
Calling CancelGroup() will discard the display items that have been pushed after calling StartGroup().

For example, inside nsDisplayBackgroundColor::CreateWebRenderCommands():
```
aBuilder.StartGroup(this);
aBuilder.PushRect(r, r, !BackfaceIsHidden(),
                  wr::ToColorF(ToDeviceColor(mColor)));
aBuilder.FinishGroup();
```

Differential Revision: https://phabricator.services.mozilla.com/D65356

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/91760460f91496fb2262242dcdf7c0deb311978c