Blink Support for Overlay Scrollbar Animation

Source/platform/scroll/Scrollbar.cpp

Index: Source/platform/scroll/Scrollbar.cpp
diff --git a/Source/platform/scroll/Scrollbar.cpp b/Source/platform/scroll/Scrollbar.cpp
index afea041f44173813c64cf9b4ceed4761658d1b6f..f1586b62e38e51620258ebc08a7b102edbc223a2 100644
--- a/Source/platform/scroll/Scrollbar.cpp
+++ b/Source/platform/scroll/Scrollbar.cpp
@@ -531,6 +531,33 @@ bool Scrollbar::isOverlayScrollbar() const
     return m_theme->usesOverlayScrollbars();
 }
 
+bool Scrollbar::isDuringStateTransitionAnimation() const
+{
+    return m_enabled && isOverlayScrollbar() && m_scrollableArea->isDuringStateTransitionAnimation();
+}
+
+double Scrollbar::stateTransitionProgress() const
+{
+    return m_stateTransitionProgress;
+}
+
+blink::WebThemeEngine::State Scrollbar::stateTransitionStartState() const
+{
+    return m_startState;
+}
+
+blink::WebThemeEngine::State Scrollbar::stateTransitionEndState() const
+{
+    return m_endState;
+}
+
+void Scrollbar::updateStateTransitionData(blink::WebThemeEngine::State startState, blink::WebThemeEngine::State endState, double progress)
+{
+    m_startState = startState;
+    m_endState = endState;
+    m_stateTransitionProgress = progress;
+    updateThumb();
+}
 bool Scrollbar::shouldParticipateInHitTesting()
 {
     // Non-overlay scrollbars should always participate in hit testing.