Add field-initializing constructors to generated mojo structs.

mojo/common/values.typemap

Index: mojo/common/values.typemap
diff --git a/mojo/common/values.typemap b/mojo/common/values.typemap
index 069c277940b2c8e083ee310c994b34d4558b5274..aa37bd09a6bd71ae55358909d9be30664f91fa33 100644
--- a/mojo/common/values.typemap
+++ b/mojo/common/values.typemap
@@ -19,7 +19,7 @@ sources = [
 
 type_mappings = [
   "mojo.common.mojom.DictionaryValue=std::unique_ptr<base::DictionaryValue>[move_only,nullable_is_same_type]",
-  "mojo.common.mojom.LegacyListValue=base::ListValue",
+  "mojo.common.mojom.LegacyListValue=base::ListValue[emplace_only]",

[yzshen1, 2017/02/14 17:21:52]

Is emplace-only a well-known term? (Is it from the standard but I don't know
about? :))

If not, does it make sense to use "non_copyable_non_movable" instead? That way
the meaning is clear at the first sight.

[Sam McNally, 2017/02/15 06:35:49]

Done.

   "mojo.common.mojom.ListValue=std::unique_ptr<base::ListValue>[move_only,nullable_is_same_type]",
   "mojo.common.mojom.Value=std::unique_ptr<base::Value>[move_only,nullable_is_same_type]",
 ]