Add a flat_map container

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index c9bb0ea2023f64dd7f7b2446119c543f8542a9c7..ee4a8cca4f60181808ea1833f10e18ec28427030 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -276,7 +276,9 @@ component("base") {
     "command_line.h",
     "compiler_specific.h",
     "containers/adapters.h",
+    "containers/flat_map.h",
     "containers/flat_set.h",
+    "containers/flat_tree.h",
     "containers/hash_tables.h",
     "containers/linked_list.h",
     "containers/mru_cache.h",
@@ -1922,7 +1924,10 @@ test("base_unittests") {
     "cancelable_callback_unittest.cc",
     "command_line_unittest.cc",
     "containers/adapters_unittest.cc",
+    "containers/container_test_utils.h",
+    "containers/flat_map_unittest.cc",
     "containers/flat_set_unittest.cc",
+    "containers/flat_tree_unittest.cc",
     "containers/hash_tables_unittest.cc",
     "containers/linked_list_unittest.cc",
     "containers/mru_cache_unittest.cc",