Fix shouldPaint issue when a composited floating iframe becomes non-self-painting

third_party/WebKit/Source/core/paint/BlockFlowPainter.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/BlockFlowPainter.h"
 
 #include "core/layout/FloatingObjects.h"
 #include "core/layout/LayoutBlockFlow.h"
 #include "core/paint/BlockPainter.h"
 #include "core/paint/LineBoxListPainter.h"
@@ skipping 35 matching lines @@
   PaintInfo floatPaintInfo(paintInfo);
   if (paintInfo.phase == PaintPhaseFloat)
     floatPaintInfo.phase = PaintPhaseForeground;
 
   for (const auto& floatingObject :
        m_layoutBlockFlow.floatingObjects()->set()) {
     if (!floatingObject->shouldPaint())
       continue;
 
     const LayoutBox* floatingLayoutObject = floatingObject->layoutObject();
+    // TODO(wangxianzhu): Should this be a DCHECK?
+    if (floatingLayoutObject->hasSelfPaintingLayer())

[chrishtr, 2017/01/25 20:47:57]

In my original patch, I removed this because it was impossible to trigger for
non-self-painting layers. Why would it trigger now?

[Xianzhu, 2017/01/25 21:03:25]

I didn't look at this closely when applying the revert. This CL needs to be
merged into M56, so I would like to take the safe side.

+      continue;
+
     // FIXME: LayoutPoint version of xPositionForFloatIncludingMargin would make
     // this much cleaner.
     LayoutPoint childPoint = m_layoutBlockFlow.flipFloatForWritingModeForChild(
         *floatingObject,
         LayoutPoint(paintOffset.x() +
                         m_layoutBlockFlow.xPositionForFloatIncludingMargin(
                             *floatingObject) -
                         floatingLayoutObject->location().x(),
                     paintOffset.y() +
                         m_layoutBlockFlow.yPositionForFloatIncludingMargin(
                             *floatingObject) -
                         floatingLayoutObject->location().y()));
     ObjectPainter(*floatingLayoutObject)
         .paintAllPhasesAtomically(floatPaintInfo, childPoint);
   }
 }
 
 }  // namespace blink