Add some class documentation to LayoutBlock

Source/core/layout/LayoutBlock.cpp

Index: Source/core/layout/LayoutBlock.cpp
diff --git a/Source/core/layout/LayoutBlock.cpp b/Source/core/layout/LayoutBlock.cpp
index f9a4e434cf19610c9211a54a49f60fc627167371..5ae6c0fc4d71b03e631f5569c9ac13e760173691 100644
--- a/Source/core/layout/LayoutBlock.cpp
+++ b/Source/core/layout/LayoutBlock.cpp
@@ -88,7 +88,18 @@ struct SameSizeAsLayoutBlock : public LayoutBox {
 
 static_assert(sizeof(LayoutBlock) == sizeof(SameSizeAsLayoutBlock), "LayoutBlock should stay small");
 
+// This map keeps track of the positioned objects associated with a containing block.
+//
+// This map is populated during layout. It is kept across layouts to handle sub-tree
+// layouts. The map could be invalidated during style change but keeping track of

[mstensho (USE GERRIT), 2015/08/31 19:28:52]

You mean "handle that we skip unchanged subtrees during layout, in such a way
that we're able to lay out deeply nested out-of-flow descendants if their
containing block got laid out"?

[Julien - ping for review, 2015/08/31 21:55:29]

Me gusta! Changed for your less hand-wavy explanation.

+// containing blocks at that time is complicated (we are in the middle of
+// recomputing the style so we can't rely on any of its information), which is why
+// it's easier to just update it for every layout.
 static TrackedDescendantsMap* gPositionedDescendantsMap = nullptr;
+
+// This map keeps track of the descendants whose 'height' is percentage associated
+// with a containing block. Like |gPositionedDescendantsMap|, it is also recomputed
+// for every layout (see the comment above about why).
 static TrackedDescendantsMap* gPercentHeightDescendantsMap = nullptr;
 
 static TrackedContainerMap* gPositionedContainerMap = nullptr;