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 35d4fbbbbf42bc5802b849f3a6507da5d2daf093..3a771f64bf005180c8f064fb030fff0584f79435 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -3698,7 +3698,8 @@ void FrameView::updateScrollbarGeometry() {
     if (oldRect != m_horizontalScrollbar->frameRect())
       setScrollbarNeedsPaintInvalidation(HorizontalScrollbar);
 
-    m_horizontalScrollbar->setEnabled(contentsWidth() > clientWidth);
+    m_horizontalScrollbar->setEnabled(contentsWidth() > clientWidth &&
+                                      !scrollbarsHidden());
     m_horizontalScrollbar->setProportion(clientWidth, contentsWidth());
     m_horizontalScrollbar->offsetDidChange();
   }
@@ -3716,7 +3717,8 @@ void FrameView::updateScrollbarGeometry() {
     if (oldRect != m_verticalScrollbar->frameRect())
       setScrollbarNeedsPaintInvalidation(VerticalScrollbar);
 
-    m_verticalScrollbar->setEnabled(contentsHeight() > clientHeight);
+    m_verticalScrollbar->setEnabled(contentsHeight() > clientHeight &&
+                                    !scrollbarsHidden());
     m_verticalScrollbar->setProportion(clientHeight, contentsHeight());
     m_verticalScrollbar->offsetDidChange();
   }
@@ -3784,6 +3786,10 @@ void FrameView::updateScrollbarsIfNeeded() {
     updateScrollbars();
 }
 
+void FrameView::didChangeScrollbarsHidden() {
+  updateScrollbars();
+}
+
 void FrameView::updateScrollbars() {
   m_needsScrollbarsUpdate = false;