From CL description: 
When associating bookmarks, sync nodes are matched to BookmarkNodes.
To match a BookmarkNode with a sync node, a BookmarkNodeFinder is
created and asked to find the child node with the matching title. When searching
for the title, BookmarkNodeFinder returns the first matching child in
its child_nodes_ map. This map is an unordered_map with undefined
iteration order. If sync node siblings share a title, the
BookmarkNodeFinder can erroneously swap their respective
BookmarkNodes.
To fix this, this CL makes the iteration order defined with
std::multimap, which stores pairs in insertion order.
This is safe since sync nodes are processed in the same
order as BookmarkNodes are stored.