Bug 1792741 - Remove nsDocElementBoxFrame. r=TYLin

After bug 1665476, it's pretty much a regular XUL box with a couple
special-cases that we can use IsRootElementStyle for. Do that, and allow
using nsFlexContainerFrame when using flexbox emulation.

Differential Revision: https://phabricator.services.mozilla.com/D158288

layout/generic/FrameClasses.py

@@ -22,7 +22,6 @@ FRAME_CLASSES = [
     Frame("nsComboboxDisplayFrame", "ComboboxDisplay", NOT_LEAF),
     Frame("nsContinuingTextFrame", "Text", LEAF),
     Frame("nsDateTimeControlFrame", "DateTimeControl", NOT_LEAF),
-    Frame("nsDocElementBoxFrame", "DocElementBox", NOT_LEAF),
     Frame("nsFieldSetFrame", "FieldSet", NOT_LEAF),
     Frame("nsFileControlFrame", "Block", LEAF),
     Frame("nsFirstLetterFrame", "Letter", NOT_LEAF),