Make WidgetHierarchyUpdatesSuspensionScope use swap instead of copy

Make WidgetHierarchyUpdatesSuspensionScope use swap instead of copy
https://bugs.webkit.org/show_bug.cgi?id=116927

Reviewed by Andreas Kling.

Merge https://chromium.googlesource.com/chromium/blink/+/1d577e1fd5c5716a06e4007e04fc3d9d0a88f5e4

This is equivalent behavior code that is unambiguously better performance. swap is O(1) time and memory,
copying a hash table is at least linear with the number of elements.

* rendering/RenderWidget.cpp:
(WebCore::WidgetHierarchyUpdatesSuspensionScope::moveWidgets):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@150872 268f45cc-cd09-0410-ab3c-d52691b4dbfc