Send scroll animation takeOver message in the same frame that it's created

third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp

Index: third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp b/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
index 85623d4c7bd72ee817a391de3c3bda140c737fdc..e8e4e3f1f596f172eb21a54d7fb645531bc449f8 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
@@ -261,9 +261,9 @@ bool ScrollAnimatorCompositorCoordinator::hasImplOnlyAnimationUpdate() const
     return !m_implOnlyAnimationAdjustment.isZero() || m_implOnlyAnimationTakeover;
 }
 
-void ScrollAnimatorCompositorCoordinator::updateCompositorAnimations()
+void ScrollAnimatorCompositorCoordinator::updateImplOnlyCompositorAnimations()
 {
-    if (!getScrollableArea()->scrollAnimatorEnabled() || !hasImplOnlyAnimationUpdate())
+    if (!hasImplOnlyAnimationUpdate())
         return;
 
     GraphicsLayer* layer = getScrollableArea()->layerForScrolling();
@@ -283,6 +283,14 @@ void ScrollAnimatorCompositorCoordinator::updateCompositorAnimations()
     m_implOnlyAnimationTakeover = false;
 }
 
+void ScrollAnimatorCompositorCoordinator::updateCompositorAnimations()
+{
+    if (!getScrollableArea()->scrollAnimatorEnabled())
+        return;
+
+    updateImplOnlyCompositorAnimations();
+}
+
 void ScrollAnimatorCompositorCoordinator::adjustAnimationAndSetScrollPosition(
     IntSize adjustment, ScrollType scrollType) {
     // Subclasses should override this and adjust the animation as necessary.
@@ -308,6 +316,11 @@ void ScrollAnimatorCompositorCoordinator::takeOverImplOnlyScrollOffsetAnimation(
 
     m_implOnlyAnimationTakeover = true;
 
+    // Update compositor animations right away to avoid skipping a frame.
+    // This imposes the constraint that this function should only be called
+    // from or after DocumentLifecycle::LifecycleState::CompositingClean state.
+    updateImplOnlyCompositorAnimations();
+
     getScrollableArea()->registerForAnimation();
 }