When accessing line boxes, use RenderBlockFlow instead of RenderBlock.

Source/core/rendering/RenderBlock.cpp

Index: Source/core/rendering/RenderBlock.cpp
diff --git a/Source/core/rendering/RenderBlock.cpp b/Source/core/rendering/RenderBlock.cpp
index 2d0e66f81a8c87a706952ff95a8683d1e81f19b1..c6ea8cd72040263ac6e7ad8fc77bb39e50d70c96 100644
--- a/Source/core/rendering/RenderBlock.cpp
+++ b/Source/core/rendering/RenderBlock.cpp
@@ -4219,21 +4219,21 @@ static bool shouldCheckLines(RenderObject* obj)
 static int getHeightForLineCount(RenderBlock* block, int l, bool includeBottom, int& count)
 {
     if (block->style()->visibility() == VISIBLE) {
-        if (block->childrenInline()) {
-            for (RootInlineBox* box = block->firstRootBox(); box; box = box->nextRootBox()) {
+        if (block->isRenderBlockFlow() && block->childrenInline()) {
+            for (RootInlineBox* box = toRenderBlockFlow(block)->firstRootBox(); box; box = box->nextRootBox()) {
                 if (++count == l)
                     return box->lineBottom() + (includeBottom ? (block->borderBottom() + block->paddingBottom()) : LayoutUnit());
             }
-        }
-        else {
+        } else {
             RenderBox* normalFlowChildWithoutLines = 0;
             for (RenderBox* obj = block->firstChildBox(); obj; obj = obj->nextSiblingBox()) {
                 if (shouldCheckLines(obj)) {
                     int result = getHeightForLineCount(toRenderBlock(obj), l, false, count);
                     if (result != -1)
                         return result + obj->y() + (includeBottom ? (block->borderBottom() + block->paddingBottom()) : LayoutUnit());
-                } else if (!obj->isFloatingOrOutOfFlowPositioned())
+                } else if (!obj->isFloatingOrOutOfFlowPositioned()) {
                     normalFlowChildWithoutLines = obj;
+                }
             }
             if (normalFlowChildWithoutLines && l == 0)
                 return normalFlowChildWithoutLines->y() + normalFlowChildWithoutLines->height();