Add infrastructure for partial layouts

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 e94a24f4835ad985a2aaff8f568278b3aa37d3b6..64ab691ccca9e24690fb0e896f5c992c8bec1561 100644
--- a/Source/core/rendering/RenderBlock.cpp
+++ b/Source/core/rendering/RenderBlock.cpp
@@ -1412,6 +1412,9 @@ void RenderBlock::layout()
     // layoutBlock().
     layoutBlock(false);
 
+    if (document()->view()->shouldStopPartialLayout())
+        return;
+
     // It's safe to check for control clip here, since controls can never be table cells.
     // If we have a lightweight clip, there can never be any overflow from children.
     if (hasControlClip() && m_overflow)
@@ -1620,6 +1623,12 @@ void RenderBlock::layoutBlock(bool relayoutChildren, LayoutUnit pageLogicalHeigh
     else
         layoutBlockChildren(relayoutChildren, maxFloatLogicalBottom);
 
+    // If layout was just for one of our children, stop early (before setting setNeedsLayout=false!)
+    if (document()->view()->shouldStopPartialLayout()) {
+        statePusher.pop();
+        return;
+    }
+
     // Expand our intrinsic height to encompass floats.
     LayoutUnit toAdd = borderAfter() + paddingAfter() + scrollbarLogicalHeight();
     if (lowestFloatLogicalBottom() > (logicalHeight() - toAdd) && expandsToEncloseOverhangingFloats())
@@ -1667,6 +1676,9 @@ void RenderBlock::layoutBlock(bool relayoutChildren, LayoutUnit pageLogicalHeigh
 
     fitBorderToLinesIfNeeded();
 
+    if (document()->view()->shouldStopPartialLayout())
+        return;
+
     if (renderView->layoutState()->m_pageLogicalHeight)
         setPageLogicalOffset(renderView->layoutState()->pageLogicalOffset(this, logicalTop()));
 
@@ -2539,6 +2551,10 @@ void RenderBlock::layoutBlockChildren(bool relayoutChildren, LayoutUnit& maxFloa
 
         // Lay out the child.
         layoutBlockChild(child, marginInfo, previousFloatLogicalBottom, maxFloatLogicalBottom);
+
+        document()->view()->checkPartialLayoutComplete(child);
+        if (document()->view()->shouldStopPartialLayout())
+            break;
     }
 
     // Now do the handling of the bottom of the block, adding in our bottom border/padding and
@@ -2603,6 +2619,9 @@ void RenderBlock::layoutBlockChild(RenderBox* child, MarginInfo& marginInfo, Lay
     if (childNeededLayout)
         child->layout();
 
+    if (document()->view()->shouldStopPartialLayout())
+        return;
+
     // Cache if we are at the top of the block right now.
     bool atBeforeSideOfBlock = marginInfo.atBeforeSideOfBlock();