Move code for locating the containing flow thread out of LayoutObject.

Source/core/layout/LayoutFlowThread.cpp

Index: Source/core/layout/LayoutFlowThread.cpp
diff --git a/Source/core/layout/LayoutFlowThread.cpp b/Source/core/layout/LayoutFlowThread.cpp
index 34e5d7793c5f95f03bd9b7239199d10e45d43fa7..d2c0a72e0a2f5d15457fe731281bc513b498d25f 100644
--- a/Source/core/layout/LayoutFlowThread.cpp
+++ b/Source/core/layout/LayoutFlowThread.cpp
@@ -43,6 +43,30 @@ LayoutFlowThread::LayoutFlowThread()
 {
 }
 
+LayoutFlowThread* LayoutFlowThread::locateFlowThreadContainingBlockOf(const LayoutObject& descendant)
+{
+    ASSERT(descendant.isInsideFlowThread());
+    LayoutObject* curr = const_cast<LayoutObject*>(&descendant);

[leviw_travelin_and_unemployed, 2015/08/11 17:51:07]

What fails in this function if you remove this const_cast?

[mstensho (USE GERRIT), 2015/08/11 19:09:18]

toLayoutFlowThread(), because the constness of input and output have to agree,
so we get the one that takes const returns const, and this methods returns (and
has to return) non-const.

Note that I'm not introducing any const cast with this patch, though. It used to
live in LayoutObject, but now I'm moving it here.

Other methods that do similar things (i.e. find something in the ancestry), such
as LayoutBlock* LayoutObject::containingBlock() const, shake off the constness
debuff by starting at their parent(). In this method we start at ourselves.

+    while (curr) {
+        if (curr->isSVG() && !curr->isSVGRoot())
+            return nullptr;
+        if (curr->isLayoutFlowThread())
+            return toLayoutFlowThread(curr);
+        LayoutObject* container = curr->container();
+        curr = curr->parent();
+        while (curr != container) {
+            if (curr->isLayoutFlowThread()) {
+                // The nearest ancestor flow thread isn't in our containing block chain. Then we
+                // aren't really part of any flow thread, and we should stop looking. This happens
+                // when there are out-of-flow objects or column spanners.
+                return nullptr;
+            }
+            curr = curr->parent();
+        }
+    }
+    return nullptr;
+}
+
 void LayoutFlowThread::removeColumnSetFromThread(LayoutMultiColumnSet* columnSet)
 {
     ASSERT(columnSet);