[sync] use std::multimap for BookmarkNodeMap

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.

R=skym@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2743713005
Cr-Commit-Position: refs/heads/master@{#456234}

components/sync_bookmarks/bookmark_model_associator.cc

@@ -94,7 +94,7 @@ class BookmarkNodeFinder {
   // Maps bookmark node titles to instances, duplicates allowed.
   // Titles are converted to the sync internal format before
   // being used as keys for the map.
-  using BookmarkNodeMap = base::hash_multimap<std::string, const BookmarkNode*>;
+  using BookmarkNodeMap = std::multimap<std::string, const BookmarkNode*>;
   using BookmarkNodeRange =
       std::pair<BookmarkNodeMap::iterator, BookmarkNodeMap::iterator>;