Optimize several things about DOM Parts [2/2]

This CL continues the optimization work, with these changes:
- Replace ClonePartsFrom() with PartRoot::CloneParts(), to centralize
  the part cloning functionality into a single place.
- Replace NodeCloningData's HeapHashMap architecture with a much
  simpler stack architecture. Also eliminate the cloning part queue,
  and replace it with a clone-as-you-go approach. With this new
  system, as a Node containing Parts is cloned, the Parts are
  iterated and cloned, validating them with IsValid first. As
  ChildNodeParts are encountered, they are cloned with both sibling
  nodes pointing to the same node, and the new ChildNodePart is pushed
  onto the part root stack. As the next_sibling is encountered, the
  ChildNodePart is updated to have the correct sibling node, and the
  part root stack is popped. All of this together achieves the same
  behavior, but with much lower speed cost.
- Part::IsValid() is cached to improve speed, and disconnected_
  becomes connected_ to avoid a `!`.
- PartRoot's cached parts list is converted from a Deque to a
  HeapVector, which is more efficient. That eliminates the ability
  to modify the front of the list, so this will adversely impact
  performance for some operations when DOMPartsAPIActivePartTracking
  is enabled (off by default).

Bug: 1453291
Change-Id: I2a2025dd4d1e0c1fa4c85a21469526f6ff1f86d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4909445
Reviewed-by: Joey Arhar <[email protected]>
Commit-Queue: Mason Freed <[email protected]>
Auto-Submit: Mason Freed <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1205329}

Author: mfreed7

dom/parts/basic-dom-part-objects.tentative.html

@@ -120,6 +120,7 @@
 
     // Test cloning of the entire DocumentPartRoot.
     const clonedPartRoot = root.clone();
+    assertEqualParts(root.getParts(),rootExpectations,[nodePart,childNodePart],'cloning a part root should not change the original');
     const clonedContainer = clonedPartRoot.rootContainer;
     assert_true(clonedPartRoot instanceof DocumentPartRoot);
     assert_true(clonedContainer instanceof (useTemplate ? DocumentFragment : Document));