Fix several crashes when nested slots are removed from a ShadowRoot

[chromium-wpt-export-bot]

There were some crashes caused by nested slots (e.g.
<slot><slot>Content</slot></slot>) being removed from the tree.
These crashes were triggered by [1], which removed Shadow DOM v0, but
my theory is that due to the old V0 shadow root code, more calls were
being made to SlotAssignment::RecalcAssignment(). Now that the V0 code
is gone, it has exposed some missing code.

Three issues are being fixed here:

1. In Node::CheckSlotChange(), while removing the inner nested slot,
   the parent_slot will have already been removed from the tree, so we
   only need to call DidSlotChange if not. This used to be a DCHECK.
2. In TreeOrderedMap::Get(), while removing a key that previously had
   more than one element, we may walk the tree and find that none of
   the pre-existing elements are present. I.e. we're in a RemoveScope.
   In this case, the key should be removed from the map.
3. In SlotAssignment::DidRemoveSlotInternal(), given Apple's submission #2 above, we can
   just early-out if the slot isn't present in the map.

I added a test for the crash conditions (variations on removing nested
named and unnamed slots), plus I added a test for the TreeOrderedMap
class, since there was none previously. The last test in the set
documents the new Get() behavior. I also tried to improve some of the
comments along the way. Finally, this CL rolls back a mitigation [2]
previously landed for this crash.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2586019
[2] https://chromium-review.googlesource.com/c/chromium/src/+/2595967

Bug: 1159328, 1159727
Change-Id: I47fbf33b2313b9ae2efe229443af6e8c9a1920a9
Cq-Do-Not-Cancel-Tryjobs: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2597040
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Yu Han <yuzhehan@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838974}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.