Promote opaque fixed position elements.

third_party/WebKit/Source/core/paint/PaintLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 2430 matching lines @@
 
     // We can't use hasVisibleContent(), because that will be true if our layoutObject is hidden, but some child
     // is visible and that child doesn't cover the entire rect.
     if (layoutObject()->style()->visibility() != EVisibility::Visible)
         return false;
 
     if (paintsWithFilters() && layoutObject()->style()->filter().hasFilterThatAffectsOpacity())
         return false;
 
     // FIXME: Handle simple transforms.
-    if (paintsWithTransform(GlobalPaintNormalPhase))
+    if (transform() && compositingState() != PaintsIntoOwnBacking)
+        return false;
+
+    if (!RuntimeEnabledFeatures::compositeOpaqueFixedPositionEnabled()
+        && layoutObject()->style()->position() == FixedPosition
+        && compositingState() != PaintsIntoOwnBacking)
         return false;
 
     // This function should not be called when layer-lists are dirty.
     // TODO(schenney) This check never hits in layout tests or most platforms, but does hit in
     // PopupBlockerBrowserTest.AllowPopupThroughContentSetting on Win 7 Test Builder.
     if (m_stackingNode->zOrderListsDirty())
         return false;
 
     // FIXME: We currently only check the immediate layoutObject,
     // which will miss many cases where additional layout objects paint
@@ skipping 519 matching lines @@
 
 void showLayerTree(const blink::LayoutObject* layoutObject)
 {
     if (!layoutObject) {
         fprintf(stderr, "Cannot showLayerTree. Root is (nil)\n");
         return;
     }
     showLayerTree(layoutObject->enclosingLayer());
 }
 #endif