Main-thread scroll if we have fixed-pos children and overflow:hidden.

Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/Source/core/page/scrolling/ScrollingCoordinator.cpp b/Source/core/page/scrolling/ScrollingCoordinator.cpp
index 894e1c2f82949d54987dec697f895c6754ed3cfd..1bfd1b4189819e4f7894d9705a92eafd8dfe18a5 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -946,8 +946,18 @@ MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons() co
 
     if (frameView->hasSlowRepaintObjects())
         reasons |= HasSlowRepaintObjects;
-    if (frameView->isScrollable() && hasVisibleSlowRepaintViewportConstrainedObjects(frameView))
+    FrameView::ScrollingReasons scrollingReasons = frameView->scrollingReasons();
+    const bool mayBeScrolledByInput = (scrollingReasons == FrameView::Scrollable);
+    const bool mayBeScrolledByScript = mayBeScrolledByInput || (scrollingReasons ==
+        FrameView::NotScrollableExplicitlyDisabled);
+
+    // TODO(awoloszyn) Currently crbug.com/304810 will let certain
+    // overflow:hidden elements scroll on the compositor thread, so we should
+    // not let this move there path as an optimization, when we have slow-repaint
+    // elements.
+    if (mayBeScrolledByScript && hasVisibleSlowRepaintViewportConstrainedObjects(frameView)) {
         reasons |= HasNonLayerViewportConstrainedObjects;
+    }
 
     return reasons;
 }