Add support for printing multicol containers, and enable it.

third_party/WebKit/Source/core/layout/LayoutView.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 20 matching lines @@
 #include "core/html/HTMLIFrameElement.h"
 #include "core/html/HTMLVideoElement.h"
 #include "core/inspector/InspectorTraceEvents.h"
 #include "core/layout/HitTestResult.h"
 #include "core/layout/LayoutFlowThread.h"
 #include "core/layout/LayoutGeometryMap.h"
 #include "core/layout/LayoutMedia.h"
 #include "core/layout/LayoutPart.h"
 #include "core/layout/LayoutQuote.h"
 #include "core/layout/LayoutScrollbarPart.h"
+#include "core/layout/ViewFragmentationContext.h"
 #include "core/layout/compositing/PaintLayerCompositor.h"
 #include "core/page/Page.h"
 #include "core/paint/PaintLayer.h"
 #include "core/paint/ViewPainter.h"
 #include "core/svg/SVGDocumentExtensions.h"
 #include "platform/TraceEvent.h"
 #include "platform/TracedValue.h"
 #include "platform/geometry/FloatQuad.h"
 #include "platform/geometry/TransformState.h"
 #include "platform/graphics/paint/PaintController.h"
@@ skipping 191 matching lines @@
     }
 
     ASSERT(!m_layoutState);
     if (!needsLayout())
         return;
 
     LayoutState rootLayoutState(pageLogicalHeight(), pageLogicalHeightChanged(), *this);
 
     m_pageLogicalHeightChanged = false;
 
+    if (m_pageLogicalHeight) {
+        if (!m_fragmentationContext)
+            m_fragmentationContext = adoptPtr(new ViewFragmentationContext(*this));
+    } else {
+        if (m_fragmentationContext)
+            m_fragmentationContext.clear();

[leviw_travelin_and_unemployed, 2015/12/08 19:00:55]

It seems a little weird to new a ViewFragmentationContext every time the view
gets height. We could avoid some thrashing if FragmentationContext had a concept
of being active.

I see we can set the pageLogicalHeight above:
if (!document().paginated())
        setPageLogicalHeight(0);

but then we may bail before getting to this code if !needsLayout(). Does this
code properly account for that case?

[mstensho (USE GERRIT), 2015/12/08 20:09:12]

Gets *page* height, not just any height. Right?

That's definitely not my intention. I only want to new it every time we enter
printing (i.e. get a page logical height). And I want to delete it when we leave
printing.
Sure, that would be easy enough to do (just add a flag, and have the descendant
multicol container ask "Do I have an enclosing ViewFragmentationContext? Am I
sure? Hey, you up there, are you active?"). But do we really need it?
I would imagine that switching from printing mode to normal mode will always
mark the view for layout, but yeah, better safe than sorry.

I'll move the code a bit further up, and piggy-back it on
shouldUsePrintingLayout() (only the root frame will normally paginate, so this
makes sense to check). I'll wait for more comments from you before doing
anything else.

+    }
+
     layoutContent();
 
     if (layoutOverflowRect() != oldLayoutOverflowRect) {
         // The document element paints the viewport background, so we need to invalidate it when
         // layout overflow changes.
         // FIXME: Improve viewport background styling/invalidation/painting. crbug.com/475115
         if (Element* documentElement = document().documentElement()) {
             if (LayoutObject* rootObject = documentElement->layoutObject())
                 rootObject->setShouldDoFullPaintInvalidation();
         }
@@ skipping 711 matching lines @@
 }
 
 void LayoutView::sendMediaPositionChangeNotifications(const IntRect& visibleRect)
 {
     for (auto& media : m_mediaForPositionNotification) {
         media->notifyPositionMayHaveChanged(visibleRect);
     }
 }
 
 } // namespace blink