Revert "[DOMStorage] Moving cached namespaces to std::map"

This reverts commit 080fe17e.

Reason for revert: The corruption is still there, but now failing on Windows.

Bug: 865393

Original change's description:
> [DOMStorage] Moving cached namespaces to std::map
> 
> There has been a bunch of memory-corruption-smelling crashes around this
> area. Changing the container to a std::map should give us more
> information about where this is happening, and maybe cause it to crash
> somewhere else.
> 
> R=​mek@chromium.org
> 
> Bug: 845400,821371,857464
> Change-Id: Ife8cb10336a4f0752db89e2bca836ed873f7c711
> Reviewed-on: https://chromium-review.googlesource.com/1129849
> Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
> Commit-Queue: Daniel Murphy <dmurph@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#573452}

TBR=dmurph@chromium.org,mek@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 845400, 821371, 857464
Change-Id: Ie120acf347713d9c6172f26082ed9b18c3cc38bc
Reviewed-on: https://chromium-review.googlesource.com/1153987
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578868}

content/renderer/dom_storage/local_storage_cached_areas.h

@@ -98,7 +98,7 @@ class CONTENT_EXPORT LocalStorageCachedAreas {
     DISALLOW_COPY_AND_ASSIGN(DOMStorageNamespace);
   };
 
-  std::map<std::string, DOMStorageNamespace> cached_namespaces_;
+  base::flat_map<std::string, DOMStorageNamespace> cached_namespaces_;
   size_t total_cache_limit_;
 
   // Not owned.