C bindings: Implement _DeepCopy() & some unittests.

mojo/public/c/bindings/struct.h

Index: mojo/public/c/bindings/struct.h
diff --git a/mojo/public/c/bindings/struct.h b/mojo/public/c/bindings/struct.h
index 67798c3c3daaa3abe27c7a81d740de2fe1bfbc80..15630571bb5566ebc04f39492e27b088e036967c 100644
--- a/mojo/public/c/bindings/struct.h
+++ b/mojo/public/c/bindings/struct.h
@@ -88,6 +88,21 @@ MojomValidationResult MojomStruct_Validate(
     uint32_t in_num_handles,
     struct MojomValidationContext* inout_context);
 
+// Creates a new copy of |in_struct| using |buffer| to allocate space.
+// Recursively creates new copies of any references from |in_struct|, and
+// updates the references to point to the new copies. This operation is useful
+// if you want to linearize |in_struct| using the buffer backed by |buffer|. If
+// there is insufficient space in the buffer, this function returns NULL and the
+// supplied buffer may be partially used.
+// Otherwise, the new copy of the struct is returned, and can then be encoded.
+// |buffer|: A mojom buffer used to allocate space for the new struct.
+// |in_type_desc|: Describes the pointer and handle fields of the mojom struct.
+// |in_struct|: The unencoded mojom struct to be copied.
+struct MojomStructHeader* MojomStruct_DeepCopy(
+    struct MojomBuffer* buffer,
+    const struct MojomTypeDescriptorStruct* in_type_desc,
+    const struct MojomStructHeader* in_struct);
+
 MOJO_END_EXTERN_C
 
 #endif  // MOJO_PUBLIC_C_BINDINGS_STRUCT_H_