Remove DisableCompositingQueryAsserts from PaintLayer codepaths

These three locations in code all have a DisableCompositingQueryAsserts
disabler in order to query some compositing state. However, none of
these operations are necessary in CompositeAfterPaint.

In PaintLayer destructor, we won't have a composited layer mapping to
clear in CAP. Enforce callers of ClearCompositedLayerMapping to have
ensured that HasCompositedLayerMapping is true, clean up an unnecessary
CAP check and more tightly scope the DisableCompositingQueryAssert to
this function.

PaintLayer::StyleDidChange queries compositing state to determine
whether or not to call SetNeedsCompositingInputsUpdate. That method is
a no-op in CAP.

PaintLayer::DirtyStackingContextZOrderLists queries compositing state
in order to mark the AncestorStackingContext's CompositedLayerMapping
as needing a graphics layer update, which also is not needed in CAP.

Bug: 1007989
Change-Id: I552b348a3bc4d6da196eaad90b05e25a6b470c97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2129090
Commit-Queue: Daniel Libby <dlibby@microsoft.com>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756315}

third_party/blink/renderer/core/paint/paint_layer.cc

@@ -221,10 +221,8 @@ PaintLayer::~PaintLayer() {
 
   // Child layers will be deleted by their corresponding layout objects, so
   // we don't need to delete them ourselves.
-  {
-    DisableCompositingQueryAsserts disabler;
+  if (HasCompositedLayerMapping())
     ClearCompositedLayerMapping(true);
-  }
 
   // Reset this flag before disposing scrollable_area_ to prevent
   // PaintLayerScrollableArea::WillRemoveScrollbar() from dirtying the z-order
@@ -2762,26 +2760,24 @@ void PaintLayer::EnsureCompositedLayerMapping() {
 }
 
 void PaintLayer::ClearCompositedLayerMapping(bool layer_being_destroyed) {
-  if (!HasCompositedLayerMapping())
-    return;
+  DCHECK(HasCompositedLayerMapping());
+  DCHECK(!RuntimeEnabledFeatures::CompositeAfterPaintEnabled());
 
+  DisableCompositingQueryAsserts disabler;
   if (layer_being_destroyed) {
-    if (!RuntimeEnabledFeatures::CompositeAfterPaintEnabled()) {
-      // The visual rects will be in a different coordinate space after losing
-      // their compositing container. Clear them before prepaint to avoid
-      // spurious layout shift reports from LayoutShiftTracker.
-      // If the PaintLayer were not being destroyed, this would happen during
-      // the compositing update (PaintLayerCompositor::UpdateIfNeeded).
-      // TODO: LayoutShiftTracker's reliance on having visual rects cleared
-      // before prepaint in the case of compositing changes is not ideal, and
-      // will not work with CompositeAfterPaint. Some transform tree changes may
-      // still produce incorrect behavior from LayoutShiftTracker (see
-      // discussion on review thread of http://crrev.com/c/1636403).
-      if (Compositor()) {
-        Compositor()
-            ->ForceRecomputeVisualRectsIncludingNonCompositingDescendants(
-                layout_object_);
-      }
+    // The visual rects will be in a different coordinate space after losing
+    // their compositing container. Clear them before prepaint to avoid
+    // spurious layout shift reports from LayoutShiftTracker.
+    // If the PaintLayer were not being destroyed, this would happen during
+    // the compositing update (PaintLayerCompositor::UpdateIfNeeded).
+    // TODO: LayoutShiftTracker's reliance on having visual rects cleared
+    // before prepaint in the case of compositing changes is not ideal, and
+    // will not work with CompositeAfterPaint. Some transform tree changes may
+    // still produce incorrect behavior from LayoutShiftTracker (see
+    // discussion on review thread of http://crrev.com/c/1636403).
+    if (Compositor()) {
+      Compositor()->ForceRecomputeVisualRectsIncludingNonCompositingDescendants(
+          layout_object_);
     }
   } else {
     // We need to make sure our decendants get a geometry update. In principle,
@@ -3154,7 +3150,7 @@ void PaintLayer::StyleDidChange(StyleDifference diff,
 
   if (diff.CompositingReasonsChanged()) {
     SetNeedsCompositingInputsUpdate();
-  } else {
+  } else if (!RuntimeEnabledFeatures::CompositeAfterPaintEnabled()) {
     // For querying stale GetCompositingState().
     DisableCompositingQueryAsserts disable;
 
@@ -3479,13 +3475,15 @@ void PaintLayer::DirtyStackingContextZOrderLists() {
   if (!stacking_context)
     return;
 
-  // This invalidation code intentionally refers to stale state.
-  DisableCompositingQueryAsserts disabler;
+  if (!RuntimeEnabledFeatures::CompositeAfterPaintEnabled()) {
+    // This invalidation code intentionally refers to stale state.
+    DisableCompositingQueryAsserts disabler;
 
-  // Changes of stacking may result in graphics layers changing size
-  // due to new contents painting into them.
-  if (auto* mapping = stacking_context->GetCompositedLayerMapping())
-    mapping->SetNeedsGraphicsLayerUpdate(kGraphicsLayerUpdateSubtree);
+    // Changes of stacking may result in graphics layers changing size
+    // due to new contents painting into them.
+    if (auto* mapping = stacking_context->GetCompositedLayerMapping())
+      mapping->SetNeedsGraphicsLayerUpdate(kGraphicsLayerUpdateSubtree);
+  }
 
   if (stacking_context->StackingNode())
     stacking_context->StackingNode()->DirtyZOrderLists();