Add a flat_map container

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index f84856de5c9533650e0ec78203c0694ac0ba38a1..d5ee3da7d63a0adbb76653779cdec5f10fa1fc9d 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",