[LayoutNG] Fix blink_unittests with LayoutNGFragmentPaint enabled.

These failures were primarily due to some missing logic within the
NGBoxFragmentPainter for CAP.

See:
https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8899518345262305936/+/steps/webkit_unit_tests__with_patch_/0/logs/Deterministic_failure:_All__x2f_BoxPainterScrollHitTestTest.ScrollHitTestOrderWithLocalBackgroundAttachment__x2f_1__status_FAILURE_/0
... as an example of the failure.

The additional CAP failure, also fails in the same way with:
--blink-enable-features=CompositeAfterPaint --disable-blink-features=LayoutNG

The code was effectively just lifted from BlockPainter.

Bug: 988015
Change-Id: I8cf273e552b2e84246eef69910c8abace9a485b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869676Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709642}

third_party/blink/renderer/core/paint/ng/ng_box_fragment_painter.cc

@@ -238,7 +238,28 @@ void NGBoxFragmentPainter::PaintInternal(const PaintInfo& paint_info) {
     info.phase = PaintPhase::kDescendantOutlinesOnly;
   } else if (ShouldPaintSelfBlockBackground(original_phase)) {
     info.phase = PaintPhase::kSelfBlockBackgroundOnly;
-    PaintObject(info, paint_offset);
+    // With CompositeAfterPaint we need to call PaintObject twice: once for the
+    // background painting that does not scroll, and a second time for the
+    // background painting that scrolls.
+    // Without CompositeAfterPaint, this happens as the main graphics layer
+    // paints the background, and then the scrolling contents graphics layer
+    // paints the background.
+    if (RuntimeEnabledFeatures::CompositeAfterPaintEnabled()) {
+      auto paint_location = ToLayoutBox(*box_fragment_.GetLayoutObject())
+                                .GetBackgroundPaintLocation();
+      if (!(paint_location & kBackgroundPaintInGraphicsLayer))
+        info.SetSkipsBackground(true);
+      PaintObject(info, paint_offset);
+      info.SetSkipsBackground(false);
+
+      if (paint_location & kBackgroundPaintInScrollingContents) {
+        info.SetIsPaintingScrollingBackground(true);
+        PaintObject(info, paint_offset);
+        info.SetIsPaintingScrollingBackground(false);
+      }
+    } else {
+      PaintObject(info, paint_offset);
+    }
     if (ShouldPaintDescendantBlockBackgrounds(original_phase))
       info.phase = PaintPhase::kDescendantBlockBackgroundsOnly;
   }
@@ -1213,6 +1234,9 @@ void NGBoxFragmentPainter::PaintAtomicInline(const PaintInfo& paint_info) {
 
 bool NGBoxFragmentPainter::IsPaintingScrollingBackground(
     const PaintInfo& paint_info) {
+  if (RuntimeEnabledFeatures::CompositeAfterPaintEnabled())
+    return paint_info.IsPaintingScrollingBackground();
+
   // TODO(layout-dev): Change paint_info.PaintContainer to accept fragments
   // once LayoutNG supports scrolling containers.
   return paint_info.PaintFlags() & kPaintLayerPaintingOverflowContents &&

third_party/blink/web_tests/FlagExpectations/enable-blink-features=CompositeAfterPaint

@@ -177,6 +177,7 @@ Bug(none) paint/invalidation/compositing/should-not-repaint-composited-filter.ht
 Bug(none) paint/invalidation/compositing/should-not-repaint-composited-opacity.html [ Failure ]
 Bug(none) paint/invalidation/compositing/should-not-repaint-move-backface-hidden.html [ Failure ]
 Bug(none) paint/invalidation/scroll/iframe-scroll-repaint.html [ Failure ]
+Bug(none) paint/invalidation/scroll/repaint-composited-child-in-scrolled-container.html [ Failure ]
 Bug(none) paint/invalidation/position/layout-state-only-positioned.html [ Failure ]
 Bug(none) paint/invalidation/position/relative-positioned-movement-repaint.html [ Failure ]