Implement overlay scrollbar fade out for non-composited scrollers.

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 5c8daac7b6ac0bc4e8d64bb70c4a93f61fe64d14..5dd7f8f4e9eec8183e2c0acd1012631aca4bd13d 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -300,9 +300,9 @@ void FrameView::dispose() {
   // from non-GC'd objects and RootFrameViewport will still have a pointer to
   // this class.
   if (m_viewportScrollableArea)
-    m_viewportScrollableArea->clearScrollAnimators();
+    m_viewportScrollableArea->clearScrollableArea();
 
-  clearScrollAnimators();
+  clearScrollableArea();
 
   // Destroy |m_autoSizeInfo| as early as possible, to avoid dereferencing
   // partially destroyed |this| via |m_autoSizeInfo->m_frameView|.
@@ -1471,6 +1471,7 @@ void FrameView::removeViewportConstrainedObject(LayoutObject* object) {
 void FrameView::viewportSizeChanged(bool widthChanged, bool heightChanged) {
   DCHECK(widthChanged || heightChanged);
 
+  showOverlayScrollbars();
   if (RuntimeEnabledFeatures::rootLayerScrollingEnabled()) {
     // The background must be repainted when the FrameView is resized, even if
     // the initial containing block does not change (so we can't rely on layout
@@ -2391,6 +2392,8 @@ bool FrameView::scrollbarsCanBeActive() const {
 }
 
 void FrameView::scrollbarVisibilityChanged() {
+  // Scrollbar enabled state is set from updateScrollbarGeometry.
+  updateScrollbarGeometry();
   LayoutViewItem viewItem = layoutViewItem();
   if (!viewItem.isNull())
     viewItem.clearHitTestCache();
@@ -3604,6 +3607,8 @@ void FrameView::updateScrollOffset(const ScrollOffset& offset,
   if (scrollDelta.isZero())
     return;
 
+  showOverlayScrollbars();
+
   if (RuntimeEnabledFeatures::rootLayerScrollingEnabled()) {
     // Don't scroll the FrameView!
     ASSERT_NOT_REACHED();
@@ -3825,10 +3830,6 @@ void FrameView::updateScrollbarsIfNeeded() {
     updateScrollbars();
 }
 
-void FrameView::didChangeScrollbarsHidden() {
-  updateScrollbars();
-}
-
 void FrameView::updateScrollbars() {
   m_needsScrollbarsUpdate = false;