Add support for printing multicol containers, and enable it.

third_party/WebKit/Source/core/layout/FragmentationContext.h

+// Copyright 2015 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.
+
+#ifndef FragmentationContext_h
+#define FragmentationContext_h
+
+#include "core/CoreExport.h"
+#include "platform/LayoutUnit.h"
+#include "platform/geometry/LayoutSize.h"
+
+namespace blink {
+
+// A fragmentation context is either established by a multicol container, or by pages when
+// printing. A fragmentation context consists of a series of fragmentainers. A fragmentainer is
+// simply a column or page, depending on the type of fragmentation context. [1]
+//
+// A couple of methods here take a |blockOffset| parameter. This is the offset from the start of the
+// fragmentation context, pretending that everything is laid out in one single strip (and not sliced
+// into pages or columns). In multicol, this is referred to as the flow thread coordinate space.
+//
+// It should be noted that a multicol container may be nested inside another fragmentation context
+// (another multicol container, or the pages when printing), although this class doesn't deal with
+// that (it's internal to the multicol implementation).
+//
+// [1] http://www.w3.org/TR/css3-break/#fragmentation-model
+class CORE_EXPORT FragmentationContext {
+public:
+    virtual ~FragmentationContext() { }
+
+    // The height of the fragmentainers may depend on the total height of the contents (column
+    // balancing), in which case false is returned if we haven't laid out yet. Otherwise, true is
+    // returned.
+    virtual bool isFragmentainerLogicalHeightKnown() = 0;
+
+    // Return the height of the fragmentainer at the specified offset. The fragmentainer height
+    // isn't necessarily uniform all across the fragmentation context.
+    virtual LayoutUnit fragmentainerLogicalHeightAt(LayoutUnit blockOffset) = 0;
+
+    // Return how much is left of the fragmentainer at the specified offset. Callers typically want
+    // this information to decide whether some piece of content fits in this fragmentainer, or if it
+    // has to push the content to the next fragmentainer.
+    virtual LayoutUnit remainingLogicalHeightAt(LayoutUnit blockOffset) = 0;
+
+    // Return the flow thread of the fragmentation context, if it is a multicol fragmentation
+    // context. Since multicol containers may be nested inside other fragmentation contexts,
+    // sometimes we need to know if it's a multicol container that we're dealing with.
+    virtual class LayoutMultiColumnFlowThread* associatedFlowThread() { return nullptr; }
+};
+
+} // namespace blink
+
+#endif // FragmentationContext_h