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 019e3cf68a705528bc7fad606dd03984414ad849..1729ee7571797d48622cfbc914af2efdfafaea9b 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.

[eae, 2015/08/28 21:02:20]

Could you please try to wrap this at 80 instead of 85 columns?

[Julien - ping for review, 2015/08/29 00:22:15]

I wrapped the following text at 85 characters instead as I didn't see the
inconsistency between the headers and the text.

+//
+// This map is cleared at the beginning of layout and populated during layout.
+// The map could be invalidated during style change but keeping track of 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
+// recompute it 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;