Reland main thread position sticky implementation.

third_party/WebKit/Source/core/frame/FrameView.cpp

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index ac7978e78fec30f3f694024d68cc30ae2103d1bd..d37c506630a9dfdfef1d643462d78d2143eec193 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -136,6 +136,7 @@ FrameView::FrameView(LocalFrame* frame)
     , m_safeToPropagateScrollToParent(true)
     , m_isTrackingPaintInvalidations(false)
     , m_scrollCorner(nullptr)
+    , m_stickyPositionObjectCount(0)
     , m_inputEventsScaleFactorForEmulation(1)
     , m_layoutSizeFixedToFrameSize(true)
     , m_didScrollTimer(this, &FrameView::didScrollTimerFired)
@@ -1601,6 +1602,14 @@ void FrameView::updateLayersAndCompositingAfterScrollIfNeeded()
     if (!hasViewportConstrainedObjects())
         return;
 
+    // Update sticky position objects which are stuck to the viewport.
+    for (const auto& viewportConstrainedObject : *m_viewportConstrainedObjects) {
+        LayoutObject* layoutObject = viewportConstrainedObject;
+        PaintLayer* layer = toLayoutBoxModelObject(layoutObject)->layer();
+        if (layoutObject->style()->position() == StickyPosition)
+            layer->updateLayerPosition();
+    }
+
     // If there fixed position elements, scrolling may cause compositing layers to change.
     // Update widget and layer positions after scrolling, but only if we're not inside of
     // layout.