In PR #909, an async mechanism for inlining link elements is introduced but it still has a bug and some negative impacts on the replayer side.

The Bug

In the stylesheet-manager.ts, when the link element loaded the styles, the manager emits an incremental mutation event to inline styles. But the mutation's parentId is wrong.

Negative Impacts

This mechanism makes elements absent in the normal snapshot stage and causes performance issues and many warnings in the console.
Here's a diagram to explain this problem.

Changes

I built this PR based on #989 so that it contains all changes of that PR. This PR has to get merged after #989.
I made some changes to the current mechanism. In the snapshot stage, the unloaded Link elements will still get serialized without inline styles. After they are loaded. an incremental mutation of attributes is appended which includes an attribute _cssText. Its value is the inlined CSS styles.

{
  source: IncrementalSource.Mutation,
  adds: [],
  removes: [],
  texts: [],
  attributes: [
    {
      id: 9,
      attributes: {
        _cssText: "body { color: pink; }",
      },
    },
  ],
}

This way, the unloaded Link elements can be inlined without causing the above-mentioned side effects.
On the replaying side, when the replayer encounters the '_cssText' attribute change and its target is the Link element, the replayer can replace the old un-inlined one with the inlined loaded Link element.
This method can make it easy to implement the TODO feature as well.

This PR can also fix #981.