Bug 1584890 Part 1 - Use nsFrameConstructorState::AddChild() to construct out-of-flow flex or grid container as the root element. r=dholbert

If `contentFrame` is out-of-flow, nsFrameConstructorState::AddChild() can
construct a placeholder frame for `contentFrame` and put the placeholder in
`frameList`.

Also, we need to use nsFrameConstructorState::GetGeometricParent() to
get the correct parent when calling InitAndRestoreFrame() for an
out-of-flow `contentFrame`. For example, if `contentFrame` has
position:fixed, its parent should be ViewportFrame, not
CanvasFrame (which is mDocElementContainingBlock).

This patch also adds reftests for position:absolute flex & grid root
element. Reftests for position:fixed root element are in the next part.

Disable crashtest 1608851.html on Android because it causes OOM crash
after landing this patch.

Differential Revision: https://phabricator.services.mozilla.com/D76205

layout/base/nsCSSFrameConstructor.cpp

@@ -2304,9 +2304,12 @@ nsIFrame* nsCSSFrameConstructor::ConstructDocElementFrame(
                     ? NS_NewGridContainerFrame
                     : NS_NewFlexContainerFrame;
     contentFrame = func(mPresShell, computedStyle);
-    InitAndRestoreFrame(state, aDocElement, mDocElementContainingBlock,
-                        contentFrame);
-    frameList = {contentFrame, contentFrame};
+    InitAndRestoreFrame(
+        state, aDocElement,
+        state.GetGeometricParent(*display, mDocElementContainingBlock),
+        contentFrame);
+    state.AddChild(contentFrame, frameList, aDocElement,
+                   mDocElementContainingBlock);
     processChildren = true;
 
     contentFrame->AddStateBits(NS_FRAME_CAN_HAVE_ABSPOS_CHILDREN);