Improvements to uses of base::SmallMap

base/containers/README.md

+# base/containers library
+
+This directory contains some STL-like containers.
+
+Things should be moved here that are generally applicable across the code base.
+Don't add things here just because you need them in one place and think others
+may someday want something similar. You can put specialized containers in
+your component's directory and we can promote them here later if we feel there
+is broad applicability.
+
+## Design and naming
+
+Containers should adhere as closely to STL as possible. Functions and behaviors
+not present in STL should only be added when they are related to the specific
+data structure implemented by the container.
+
+For STL-like containers our policy is that they should use STL-like naming even
+when it may conflict with the style guide. So functions and class names should
+be lower case with underscores. Non-STL-like classes and functions should use
+Google naming. Be sure to use the base namespace.