Implement findAnchor and computeRelativeOffset in ScrollAnchor.

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..070654f048df9d1e3c1743b8b34695e3a5af6c5f 100644
--- a/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
+++ b/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
@@ -4,6 +4,10 @@
 
 #include "core/layout/ScrollAnchor.h"
 
+#include "core/frame/FrameView.h"
+#include "core/layout/LayoutBox.h"
+#include "core/layout/LayoutView.h"
+#include "core/paint/PaintLayerScrollableArea.h"
 #include "platform/scroll/ScrollableArea.h"
 #include "wtf/Assertions.h"
 
@@ -21,16 +25,41 @@ ScrollAnchor::~ScrollAnchor()
 {
 }
 
+static LayoutBox* scrollerLayoutBox(const ScrollableArea* scroller)
+{
+    LayoutBox* box = scroller->isFrameView()
+        ? toFrameView(scroller)->layoutView()
+        : &toPaintLayerScrollableArea(scroller)->box();
+    ASSERT(box);
+    return box;
+}
+
 static LayoutObject* findAnchor(const ScrollableArea* scroller)
 {
-    // TODO(skobes): implement.
-    return nullptr;
+    LayoutBox* scrollerBox = scrollerLayoutBox(scroller);
+
+    FloatRect absoluteVisibleRect = scroller->isFrameView()
+        ? scroller->visibleContentRect()
+        : scrollerBox->localToAbsoluteQuad(
+            FloatQuad(FloatRect(scrollerBox->overflowClipRect(LayoutPoint())))).boundingBox();
+
+    LayoutObject* child = scrollerBox->nextInPreOrder(scrollerBox);
+    LayoutObject* candidate = nullptr;
+    while (child) {
+        // TODO(skobes): Compute scroller-relative bounds instead of absolute bounds.
+        FloatRect childRect = child->absoluteBoundingBoxFloatRect();
+        if (absoluteVisibleRect.intersects(childRect))
+            candidate = child;
+        if (absoluteVisibleRect.contains(childRect))
+            break;
+        child = child->nextInPreOrder(scrollerBox);
+    }
+    return candidate;
 }
 
 static DoublePoint computeRelativeOffset(const ScrollableArea* scroller, const LayoutObject* layoutObject)
 {
-    // TODO(skobes): implement.
-    return DoublePoint();
+    return DoublePoint(layoutObject->localToAbsolute() - scrollerLayoutBox(scroller)->localToAbsolute());
 }
 
 void ScrollAnchor::save()