Disable overlay scrollbars in Blink when hidden by the compositor.

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 8a3bfdab13304af8192d6e487225889e4d25b60f..b0900d71a5e93f8558fed8ad92d2d2b119db4895 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -3736,7 +3736,8 @@ void FrameView::updateScrollbarGeometry() {
     if (oldRect != horizontalScrollbar()->frameRect())
       setScrollbarNeedsPaintInvalidation(HorizontalScrollbar);
 
-    horizontalScrollbar()->setEnabled(contentsWidth() > clientWidth);
+    horizontalScrollbar()->setEnabled(contentsWidth() > clientWidth &&
+                                      !scrollbarsHidden());
     horizontalScrollbar()->setProportion(clientWidth, contentsWidth());
     horizontalScrollbar()->offsetDidChange();
   }
@@ -3754,7 +3755,8 @@ void FrameView::updateScrollbarGeometry() {
     if (oldRect != verticalScrollbar()->frameRect())
       setScrollbarNeedsPaintInvalidation(VerticalScrollbar);
 
-    verticalScrollbar()->setEnabled(contentsHeight() > clientHeight);
+    verticalScrollbar()->setEnabled(contentsHeight() > clientHeight &&
+                                    !scrollbarsHidden());
     verticalScrollbar()->setProportion(clientHeight, contentsHeight());
     verticalScrollbar()->offsetDidChange();
   }
@@ -3822,6 +3824,10 @@ void FrameView::updateScrollbarsIfNeeded() {
     updateScrollbars();
 }
 
+void FrameView::didChangeScrollbarsHidden() {
+  updateScrollbars();
+}
+
 void FrameView::updateScrollbars() {
   m_needsScrollbarsUpdate = false;