mojo: Fix potential name collision in unions C++ bindings

Changes parameter {{field.name}} to "value" to avoid
conflict with internally declared variable "result". This prevents
build failure when {{field.name}} is "result".

Bug: None
Change-Id: I2a462eb07ead8adaa4d79ce4cea7c0cfb81db441
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4987067
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Imran Ziad <imranziad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1217447}

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

@@ -22,9 +22,9 @@ class {{export_attribute}} {{union.name}} {
   // Construct an instance holding |{{field.name}}|.
   static {{union.name}}Ptr
   New{{field.name|under_to_camel}}(
-      {{field.kind|cpp_wrapper_param_type_new}} {{field.name}}) {
+      {{field.kind|cpp_wrapper_param_type_new}} value) {
     auto result = {{union.name}}Ptr(absl::in_place);
-    result->set_{{field.name}}(std::move({{field.name}}));
+    result->set_{{field.name}}(std::move(value));
     return result;
   }
 {%- endfor %}