Mojo: Add operator== for struct/union classes

We generate Equals() methods for struct and union wrapper classes.
There's not really a good reason to do this instead of operator==, and
operator== plays nicely with gmock matchers.

This adds operator== to do the same thing as Equals.

Bug: 735302
Change-Id: I4ab8c70aa4ba0276aec7a2f64734c710a090d6f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2456148
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815303}

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

@@ -59,6 +59,9 @@ class {{export_attribute}} {{struct.name}} {
   template <typename T, {{struct.name}}::EnableIfSame<T>* = nullptr>
   bool Equals(const T& other) const;
 
+  template <typename T, {{struct.name}}::EnableIfSame<T>* = nullptr>
+  bool operator==(const T& rhs) const { return Equals(rhs); }
+
 {%- if struct|is_hashable %}
   size_t Hash(size_t seed) const;
 {%- endif %}

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

@@ -49,6 +49,11 @@ class {{export_attribute}} {{union.name}} {
                 T, {{union.name}}>::value>::type* = nullptr>
   bool Equals(const T& other) const;
 
+  template <typename T,
+            typename std::enable_if<std::is_same<
+                T, {{union.name}}>::value>::type* = nullptr>
+  bool operator==(const T& rhs) const { return Equals(rhs); }
+
 {%- if union|is_hashable %}
   size_t Hash(size_t seed) const;
 {%- endif %}