Get rid of old flow thread comments and asserts in accumulateOffsetTowardsAncestor().

Source/core/paint/DeprecatedPaintLayer.cpp

Index: Source/core/paint/DeprecatedPaintLayer.cpp
diff --git a/Source/core/paint/DeprecatedPaintLayer.cpp b/Source/core/paint/DeprecatedPaintLayer.cpp
index 3823e454e11d1101fab898758d27abdbd0584f40..6ffcb5f0ac9d92f79a4aa844f9fab1a1be795ed9 100644
--- a/Source/core/paint/DeprecatedPaintLayer.cpp
+++ b/Source/core/paint/DeprecatedPaintLayer.cpp
@@ -1266,11 +1266,6 @@ static inline const DeprecatedPaintLayer* accumulateOffsetTowardsAncestor(const
     const LayoutBoxModelObject* layoutObject = layer->layoutObject();
     EPosition position = layoutObject->style()->position();
 
-    // FIXME: Positioning of out-of-flow(fixed, absolute) elements collected in a LayoutFlowThread

[mstensho (USE GERRIT), 2015/06/22 19:56:44]

Comment from the early development stages in WebKit. Addressed in Blink some
long time ago, with DeprecatedPaintLayer::visualOffsetFromAncestor() et al.

-    // may need to be revisited in a future patch.
-    // If the fixed layoutObject is inside a LayoutFlowThread, we should not compute location using localToAbsolute,

[mstensho (USE GERRIT), 2015/06/22 19:56:44]

This is a comment that survived the https://codereview.chromium.org/616453002
cleanup, but shouldn't have. There used to be a type of flow threads that acted
as containers for fixed positioned descendants (something to do with CSS
regions), but this is long gone.

-    // since localToAbsolute maps the coordinates from flow thread to column set coordinates and column sets can be
-    // positioned in a completely different place in the viewport (LayoutView).
     if (position == FixedPosition && (!ancestorLayer || ancestorLayer == layoutObject->view()->layer())) {
         // If the fixed layer's container is the root, just add in the offset of the view. We can obtain this by calling
         // localToAbsolute() on the LayoutView.
@@ -1279,9 +1274,6 @@ static inline const DeprecatedPaintLayer* accumulateOffsetTowardsAncestor(const
         return ancestorLayer;
     }
 
-    // For the fixed positioned elements inside a layout flow thread, we should also skip the code path below

[mstensho (USE GERRIT), 2015/06/22 19:56:44]

Leftovers from https://codereview.chromium.org/616453002 , again.

-    // Otherwise, for the case of ancestorLayer == rootLayer and fixed positioned element child of a transformed
-    // element in layout flow thread, we will hit the fixed positioned container before hitting the ancestor layer.
     if (position == FixedPosition) {
         // For a fixed layers, we need to walk up to the root to see if there's a fixed position container
         // (e.g. a transformed layer). It's an error to call convertToLayerCoords() across a layer with a transform,
@@ -1332,9 +1324,6 @@ static inline const DeprecatedPaintLayer* accumulateOffsetTowardsAncestor(const
         parentLayer = layer->parent();
         bool foundAncestorFirst = false;
         while (parentLayer) {
-            // LayoutFlowThread is a positioned container, child of LayoutView, positioned at (0,0).

[mstensho (USE GERRIT), 2015/06/22 19:56:44]

CSS regions stuff. Those flow threads used to be direct children of LayoutView,
and be a containing block for fixed positioned descendants.

-            // This implies that, for out-of-flow positioned elements inside a LayoutFlowThread,
-            // we are bailing out before reaching root layer.
             if (parentLayer->isPositionedContainer())
                 break;
 
@@ -1346,10 +1335,6 @@ static inline const DeprecatedPaintLayer* accumulateOffsetTowardsAncestor(const
             parentLayer = parentLayer->parent();
         }
 
-        // We should not reach LayoutView layer past the LayoutFlowThread layer for any

[mstensho (USE GERRIT), 2015/06/22 19:56:44]

Nothing incorrect in this assertion as such, but it just feels like a strange
place to suddenly start worrying about flow threads. Another relic from the CSS
regions days.

-        // children of the LayoutFlowThread.
-        ASSERT(!layoutObject->flowThreadContainingBlock() || parentLayer != layoutObject->view()->layer());
-
         if (foundAncestorFirst) {
             // Found ancestorLayer before the abs. positioned container, so compute offset of both relative
             // to enclosingPositionedAncestor and subtract.