Bug 1799425. Only set checkboard flag when adding background color item for root content document. r=emilio

Webrender uses this flag as a hint to separate mix blend containers so it should only be present on the root content document.

Differential Revision: https://phabricator.services.mozilla.com/D162046

layout/base/PresShell.cpp

@@ -5428,7 +5428,8 @@ void PresShell::AddCanvasBackgroundColorItem(
   if (!addedScrollingBackgroundColor || forceUnscrolledItem) {
     nsDisplaySolidColor* item = MakeDisplayItem<nsDisplaySolidColor>(
         aBuilder, aFrame, aBounds, bgcolor);
-    if (addedScrollingBackgroundColor) {
+    if (addedScrollingBackgroundColor &&
+        mPresContext->IsRootContentDocumentCrossProcess()) {
       item->SetIsCheckerboardBackground();
     }
     AddDisplayItemToBottom(aBuilder, aList, item);