Merge dirty relayout boundaries after RenderObject.invokeLayoutCallback

[LongCatIsLooong]

An example where widget 3's subtree will be laid out twice in one frame:

InheritedWidget
  |
LayoutBuilder with an anonymous function as builder 
  |
Widget 1, which prevents incoming constraints from forcing a tree walk 
  |
Widget 2 which produces a relayout boundary
  |
Widget 3, which produces a relayout boundary, and also depends on the InheritedWidget to do layout correctly.

When the InheritedWidget changes, widget 3's render object will be marked as dirty so the initial dirty list will be:
[RenderLayoutBuilder, RenderWidget3].

After PipelineOwner.flushLayout starts, the LayoutBuilder rebuilds, dirtying Widget 2 and causing it to produce different constraints for widget 3. But since
PipelineOwner.flushLayout is already running so the list of dirty relayout boundaries is already locked. So RenderWidget3 still gets to do layout before RenderWidget2, using outdated constraints.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[goderbauer]

If the node causing _shouldMergeDirtyNodes to be set to true is the last node in dirtyNodes this could cause an unnecessary node merging, no?

Scenario: Line 1000 causes _shouldMergeDirtyNodes to go to true and since there are no more nodes in dirtyNodes, we exit the for loop. Since there are new node in _nodesNeedingLayout we do another run through the body of the while: We sort the nodes, enter the for loop again and since _shouldMergeDirtyNodes is still true, we do an unnecessary node merge.

[goderbauer]

LGTM