Use ScrollAnchor in FrameView and PaintLayerScrollableArea.

third_party/WebKit/Source/core/layout/ScrollAnchor.cpp

Index: third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
diff --git a/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp b/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
index 0b3212850f6eb4a78d7d6013a92dcba244a55370..3242c0d94d87e53a6851496dfa2c6488d5e56e30 100644
--- a/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
+++ b/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
@@ -4,6 +4,7 @@
 
 #include "core/layout/ScrollAnchor.h"
 
+#include "core/layout/LayoutObject.h"
 #include "platform/scroll/ScrollableArea.h"
 #include "wtf/Assertions.h"
 
@@ -35,11 +36,13 @@ static DoublePoint computeRelativeOffset(const ScrollableArea* scroller, const L
 
 void ScrollAnchor::save()
 {
-    if (!m_anchorObject)
+    if (!m_anchorObject) {
         m_anchorObject = findAnchor(m_scroller);
-
-    if (m_anchorObject)
-        m_savedRelativeOffset = computeRelativeOffset(m_scroller, m_anchorObject);
+        if (!m_anchorObject)
+            return;
+        m_anchorObject->setIsScrollAnchorObject();
+    }
+    m_savedRelativeOffset = computeRelativeOffset(m_scroller, m_anchorObject);

[ojan, 2016/02/05 11:01:52]

I think this could be inside the if clause above as well? Any time something
changes that could affect this offset, we have to clear the anchorObject
anyways. So, with that, you could just have an if (m_anchorObject) early return.

[skobes, 2016/02/05 17:52:31]

In theory, yes, but that seems brittle to me.  Suppose something changes that
affects this offset, but we fail to clear the anchor.  If save() always
recalculates the relative offset, we will just anchor to a poorly-chosen node. 
But if we cache the offset we may *create* a visible jump where there wouldn't
have been one, which is much worse.

[ojan, 2016/02/06 00:09:27]

I can't think of any scenario in which it would make sense to keep the anchor
but not the offset. If we had such a thing, it would represent a real bug that
we need to fix anyways, so this extra defensiveness would just mask the bug.
computeRelativeOffset will be doing real work on each layout. It's not the end
of the world, but it's one of a thousand cuts that isn't serving a purpose.

 }
 
 void ScrollAnchor::restore()
@@ -52,4 +55,13 @@ void ScrollAnchor::restore()
         m_scroller->setScrollPosition(m_scroller->scrollPositionDouble() + adjustment, AnchoringScroll);
 }
 
+void ScrollAnchor::clear()
+{
+    LayoutObject* anchorObject = m_anchorObject;
+    m_anchorObject = nullptr;
+
+    if (anchorObject)
+        anchorObject->maybeClearIsScrollAnchorObject();
+}
+
 } // namespace blink