Make WidgetHierarchyUpdatesSuspensionScope use swap instead of copy

Source/core/rendering/RenderWidget.cpp

Index: Source/core/rendering/RenderWidget.cpp
diff --git a/Source/core/rendering/RenderWidget.cpp b/Source/core/rendering/RenderWidget.cpp
index c4f24f2dfb41baff589d81c81a19f37d0db8240e..3f1a80056d52d87391f3282ad1118e2bcc0e0902 100644
--- a/Source/core/rendering/RenderWidget.cpp
+++ b/Source/core/rendering/RenderWidget.cpp
@@ -53,8 +53,8 @@ WidgetHierarchyUpdatesSuspensionScope::WidgetToParentMap& WidgetHierarchyUpdates
 
 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();