Change mojo enums to be scoped enums in the generated C++ bindings.

mojo/public/tools/bindings/generators/cpp_templates/enum_macros.tmpl

 {#---
   Macro for enum definition, and the declaration of associated functions.
   `is_static` is relevant if this enum declaration is defined within a class, in
   which case associated functions need to be static.
 ---#}
 {%- macro enum_decl(enum, is_static=false) %}
-enum {{enum.name}} : int32_t {
+enum class {{enum.name}} : int32_t {
 {%- for field in enum.fields %}
 {%-    if field.value %}
-  {{enum.name|to_all_caps}}_{{field.name}} = {{field.value|expression_to_text}},
+  {{field.name}} = {{field.value|expression_to_text}},
 {%-    else %}
-  {{enum.name|to_all_caps}}_{{field.name}},
+  {{field.name}},
 {%-    endif %}
 {%- endfor %}
 };
 {{is_valid_enum_decl(enum, is_static)}}
 {%- endmacro %}
 
 {#--- macros for the declaration & definitions of enum-associated functions.
       Namely:
   * {enum_name}_IsValidValue:  returns true if the given enum has a valid value
                                for this generated version of enum.
 ---#}
 
 {%- macro is_valid_enum_decl(enum, is_static=false) %}
 {% if is_static %}static {% endif -%}
 bool {{enum.name}}_IsValidValue({{enum.name}} value);
 {%- endmacro %}
 
+{%- macro enum_stream_operator(enum) %}
+inline std::ostream& operator<<(std::ostream& os, {{enum|get_name_for_kind}} value) {
+  switch(value) {
+{%- for _, values in enum.fields|groupby('numeric_value') %}
+    case {{enum|get_name_for_kind}}::{{values[0].name}}:
+      return os << "{{enum|get_name_for_kind}}::
+{%-   if values|length > 1 -%}
+      {{'{'}}
+{%-   endif -%}
+      {{values|map(attribute='name')|join(', ')}}
+{%-   if values|length > 1 -%}
+      {{'}'}}
+{%-   endif -%}
+      ";
+{%- endfor %}
+    default:
+      return os << "Unknown {{enum|get_name_for_kind}} value: " << static_cast<int32_t>(value);
+  }
+}
+{%- endmacro %}
+
 {%- macro is_valid_enum_def(enum, class_name = '') %}
 {% if class_name != '' -%}
 // static
 bool {{class_name}}::
 {%- else -%}
 {{"bool "}}
 {%- endif -%}
 {{enum.name}}_IsValidValue({{enum.name}} value) {
   switch (static_cast<int32_t>(value)) {
 {%- for enum_field in enum.fields|groupby('numeric_value') %}
     case {{enum_field[0]}}:
 {%- endfor %}
       return true;
   }
   return false;
 }
 {%- endmacro %}