Use the doc rect to figure out how many pages there are

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

Index: third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
index fd8d01590aaed0b1bff70647fbcf87ff17be4011..16d00347549e98e37302e81d4e9a4b176be9e98a 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
@@ -485,10 +485,10 @@ PaintLayerPainter::PaintResult PaintLayerPainter::paintLayerWithTransform(Graphi
     if (isFixedPosObjectInPagedMedia) {
         // "For paged media, boxes with fixed positions are repeated on every page."
         // - https://www.w3.org/TR/2011/REC-CSS2-20110607/visuren.html#fixed-positioning
-        ASSERT(view->firstChild() && view->firstChild()->isLayoutBlock());
-        int pages = static_cast<int>(ceilf(toLayoutBlock(view->firstChild())->logicalHeight() / view->pageLogicalHeight()));
+        // TODO(rhogan): Figure out the correct behaviour for vertical writing modes.

[Xianzhu, 2016/06/08 21:14:06]

Can you address the vertical writing mode issue (which is introduced by this
patch) in this patch to avoid regression? I think you can just use:
  int documentLogicalHeight = isHorizontal ? view->documentRect().height() :
view->documentRect().width();

+        unsigned pages = ceilf(view->documentRect().height() / view->pageLogicalHeight());
         LayoutPoint paginationOffset;
-        for (int i = 0; i < pages; i++) {
+        for (unsigned i = 0; i < pages; i++) {
             PaintLayerFragment fragment;
             fragment.backgroundRect = paintingInfo.paintDirtyRect;
             fragment.paginationOffset = paginationOffset;