This is a follow up of Bug 1641085 Part 1. That is, I'd like to get rid of nsFrameList's copy constructor and copy assignment operator so that the frames' ownership and ownership transfer is clearer.

This patch doesn't change behavior, and eliminates copy construction of
nsFrameList via utilizing const-references to store the return value of
GetChildList(). This is the first step toward making nsFrameList a move-only
class.

This patch doesn't change behavior.

AbsoluteFrameList is a derived class of nsFrameList, so we change it a move-only
class in order to nsFrameList move-only in Part 3.

Some detail of this patch:

• Define move constructor and move assignment operator for AbsoluteFrameList.
  This effectively disables the auto generated copy constructor and copy
  assignement operator.

• Initialize nsFrameConstructorSaveState's member variables in class definition,
  and remove its constructor.

• Remove mSavedFixedList since we can rewire the logic in
  ~nsFrameConstructorSaveState and PushAbsoluteContainingBlock() to make it
  redundant.

• Make self-assignment correct in nsFrameList's move assignment operator.

Depends on D160013

This patch doesn't change behavior.

Depends on D160014

Depends on D160015

Backed out for causing high frequency ThreadSanitizer failures.

Backout link: https://hg.mozilla.org/integration/autoland/rev/d1b7ddc7c3b5c4951354838afe3e9475533436fe

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&revision=e55bef9aa4881d46e17b9c250ae9440eb0d0e53a&selectedTaskRun=DnEV3gXgRemM4nQhyA7xaQ.0

Failure log: https://treeherder.mozilla.org/logviewer?job_id=394182599&repo=autoland&lineNumber=3003

https://hg.mozilla.org/mozilla-central/rev/18e0c89e758c
https://hg.mozilla.org/mozilla-central/rev/3f443cbb5bdd
https://hg.mozilla.org/mozilla-central/rev/800047b26683
https://hg.mozilla.org/mozilla-central/rev/59a4b2b70632

(In reply to Pulsebot from comment #5)

Pushed by aethanyc@gmail.com:
https://hg.mozilla.org/integration/autoland/rev/9a3f104b78c5
Part 1 - Delete nsFrameList's copy constructor. r=layout-reviewers,emilio
https://hg.mozilla.org/integration/autoland/rev/360739dfb6d1
Part 2 - Make AbsoluteFrameList a move-only class; improve
nsFrameConstructorSaveState and PushAbsoluteContainingBlock(). r=emilio
https://hg.mozilla.org/integration/autoland/rev/515757be2f7e
Part 3 - Delete nsFrameList's copy assigment operator. r=emilio
https://hg.mozilla.org/integration/autoland/rev/e55bef9aa488
Part 4 - Rename AbsoluteFrameList::containingBlock to mContainingBlock to
match coding style. r=emilio,layout-reviewers

== Change summary for alert #35821 (as of Thu, 27 Oct 2022 15:00:44 GMT) ==

Regressions:

Improvements:

For up to date results, see: https://treeherder.mozilla.org/perfherder/alerts?id=35821