Make WidgetHierarchyUpdatesSuspensionScope use swap instead of copy

Source/core/rendering/RenderWidget.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2006, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 35 matching lines @@
 unsigned WidgetHierarchyUpdatesSuspensionScope::s_widgetHierarchyUpdateSuspendCount = 0;
 
 WidgetHierarchyUpdatesSuspensionScope::WidgetToParentMap& WidgetHierarchyUpdatesSuspensionScope::widgetNewParentMap()
 {
     DEFINE_STATIC_LOCAL(WidgetToParentMap, map, ());
     return map;
 }
 
 void WidgetHierarchyUpdatesSuspensionScope::moveWidgets()
 {
-    WidgetToParentMap map = widgetNewParentMap();
-    widgetNewParentMap().clear();
+    WidgetToParentMap map;
+    widgetNewParentMap().swap(map);

[eseidel, 2013/05/23 03:12:22]

This extends the lifetime of widgets in this map to the end of this function
instead of being cleared immediately, no?  I'm not sure this matters... but it
may change behavior?

[dmichael (off chromium), 2013/05/23 15:52:58]

Note how the old code copies the map in line 56. The map's type is:
    typedef HashMap<RefPtr<Widget>, FrameView*> WidgetToParentMap;
So the old code copied the RefPtr<Widget>s and the FrameView raw pointers to the
new local hash map, which lives until the end of scope.

When we call swap, the static map from widgetNewParentMap gets the contents of
the empty |map|, so it's the same as clearing it. |map| gets the contents of the
static map from widgetNewParentMap. Because the map just holds pointer types,
this has the same effect as copying (without the memory and time overhead of
filling a second hash map, copying ptrs, and extra refcounting). The behavior
should be equivalent. Please let me know if I missed something.

     WidgetToParentMap::iterator end = map.end();
     for (WidgetToParentMap::iterator it = map.begin(); it != end; ++it) {
         Widget* child = it->key.get();
         ScrollView* currentParent = child->parent();
         FrameView* newParent = it->value;
         if (newParent != currentParent) {
             if (currentParent)
                 currentParent->removeChild(child);
             if (newParent)
                 newParent->addChild(child);
@@ skipping 307 matching lines @@
 CursorDirective RenderWidget::getCursor(const LayoutPoint& point, Cursor& cursor) const
 {
     if (widget() && widget()->isPluginView()) {
         // A plug-in is responsible for setting the cursor when the pointer is over it.
         return DoNotSetCursor;
     }
     return RenderReplaced::getCursor(point, cursor);
 }
 
 } // namespace WebCore