Fix scroll origin, overflow rects, and coordinate flipping for flexbox.

third_party/WebKit/Source/core/layout/LayoutBox.h

Index: third_party/WebKit/Source/core/layout/LayoutBox.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutBox.h b/third_party/WebKit/Source/core/layout/LayoutBox.h
index bff6c3b63427dd7bc59b3cfa17e2d73c9ac43e5c..c08f774a7f639b7ef02a5ab18bbb862058541550 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBox.h
+++ b/third_party/WebKit/Source/core/layout/LayoutBox.h
@@ -380,6 +380,13 @@ public:
     LayoutRect selfVisualOverflowRect() const { return m_overflow ? m_overflow->selfVisualOverflowRect() : borderBoxRect(); }
     LayoutRect contentsVisualOverflowRect() const { return m_overflow ? m_overflow->contentsVisualOverflowRect() : LayoutRect(); }
 
+    // These methods don't mean the box *actually* has top/left overflow.  They mean that

[cbiesinger, 2016/05/11 19:44:00]

maybe rename to canOverflowTo{Top,Left}?

+    // *if* the box overflows, it will overflow to the top/left rather than the bottom/right.
+    // This happens when child content is laid out right-to-left (e.g. direction:rtl) or
+    // or bottom-to-top (e.g. direction:rtl writing-mode:vertical-rl).
+    virtual bool hasTopOverflow() const;

[leviw_travelin_and_unemployed, 2016/05/10 17:52:42]

The name still strikes me as confusing (seems like if we called them
overflowFlowsLeft/Top or something you may not need as much comment), but I
won't block an otherwise amazing patch over this.

+    virtual bool hasLeftOverflow() const;
+
     void addLayoutOverflow(const LayoutRect&);
     void addSelfVisualOverflow(const LayoutRect&);
     void addContentsVisualOverflow(const LayoutRect&);
@@ -871,6 +878,8 @@ public:
 
     virtual bool needsPreferredWidthsRecalculation() const;
 
+    // See README.md for an explanation of scroll origin.
+    virtual IntSize originAdjustmentForScrollbars() const;
     IntSize scrolledContentOffset() const;
 
     // Maps a rect in scrolling contents space to box space and apply overflow clip if needed.