Mojo cpp bindings: support Clone() for structs and arrays which don't contain handles.

mojo/public/cpp/bindings/struct_ptr.h

Index: mojo/public/cpp/bindings/struct_ptr.h
diff --git a/mojo/public/cpp/bindings/struct_ptr.h b/mojo/public/cpp/bindings/struct_ptr.h
index 58b6cec87f9c1d96d1ed9101eac7835c70a22f4d..46729dd062e1c7ff983d93e6bd7d2b5bd8f0feaf 100644
--- a/mojo/public/cpp/bindings/struct_ptr.h
+++ b/mojo/public/cpp/bindings/struct_ptr.h
@@ -67,6 +67,13 @@ class StructPtr {
     std::swap(ptr_, other->ptr_);
   }
 
+  // Please note that calling this method will fail compilation if the value
+  // type |Struct| doesn't have a Clone() method defined (which usually means
+  // that it contains Mojo handles).
+  StructPtr Clone() const {
+    return is_null() ? StructPtr() : ptr_->Clone();
+  }
+
  private:
   typedef Struct* StructPtr::*Testable;
 
@@ -132,6 +139,10 @@ class InlinedStructPtr {
     std::swap(is_null_, other->is_null_);
   }
 
+  InlinedStructPtr Clone() const {
+    return is_null() ? InlinedStructPtr() : value_.Clone();
+  }
+
  private:
   typedef Struct InlinedStructPtr::*Testable;