Bug 1824886 - Rewrite AnonymousContent to use a shadow tree. r=smaug,TYLin,sfoster,devtools-reviewers,nchevobbe

Subtle things:

 * We now have shadow trees in NAC, inception! Only a couple lines of
   code in the style system had to be changed to match :host rules and
   so properly.

 * Had to make highlighters.css contentaccessible, because otherwise we
   can't load it from the shadow tree. I don't think it's a big deal.

 * I removed some of the code from highlighters.css that claimed that
   stuff inherited from the html element. That's just no longer true.

 * Had to switch from `setAttribute("style", ...)` to `.style = ...;`.
   This is needed because CSSOM from chrome code bypasses CSP (as
   AnonymousContent did), but setAttribute() doesn't, see bug 1424474.

Differential Revision: https://phabricator.services.mozilla.com/D173998

layout/base/PresShell.cpp

@@ -903,11 +903,12 @@ void PresShell::Init(nsPresContext* aPresContext, nsViewManager* aViewManager) {
   // Add the preference style sheet.
   UpdatePreferenceStyles();
 
-  bool accessibleCaretEnabled =
+  const bool accessibleCaretEnabled =
       AccessibleCaretEnabled(mDocument->GetDocShell());
   if (accessibleCaretEnabled) {
     // Need to happen before nsFrameSelection has been set up.
     mAccessibleCaretEventHub = new AccessibleCaretEventHub(this);
+    mAccessibleCaretEventHub->Init();
   }
 
   mSelection = new nsFrameSelection(this, nullptr, accessibleCaretEnabled);