Allow Mojo structs as map keys

mojo/public/interfaces/bindings/tests/data/validation/conformance_mthd20_good.data

Index: mojo/public/interfaces/bindings/tests/data/validation/conformance_mthd20_good.data
diff --git a/mojo/public/interfaces/bindings/tests/data/validation/conformance_mthd20_good.data b/mojo/public/interfaces/bindings/tests/data/validation/conformance_mthd20_good.data
new file mode 100644
index 0000000000000000000000000000000000000000..df2b69abcd5100f0ca20ebbe2153025dfd9d6182
--- /dev/null
+++ b/mojo/public/interfaces/bindings/tests/data/validation/conformance_mthd20_good.data
@@ -0,0 +1,47 @@
+[dist4]message_header  // num_bytes
+[u4]0                  // version
+[u4]0                  // interface ID
+[u4]20                 // name
+[u4]0                  // flags
+[u4]0                  // padding
+[anchr]message_header
+
+[dist4]method20_params  // num_bytes
+[u4]0                   // version
+[dist8]map_data_ptr     // param0
+[anchr]method20_params
+
+[anchr]map_data_ptr
+[dist4]map_data_struct_header  // num_bytes
+[u4]0                          // version
+[dist8]key_array_ptr
+[dist8]value_array_ptr
+[anchr]map_data_struct_header
+
+[anchr]key_array_ptr
+[dist4]key_array_member  // num_bytes
+[u4]2                    // num_elements
+[dist8]key_struct_a_1
+[dist8]key_struct_a_2
+[anchr]key_array_member
+
+[anchr]key_struct_a_1
+[dist4]key_struct_a_1_member  // num_bytes
+[u4]0                         // version
+[u8]1234                      // i
+[anchr]key_struct_a_1_member
+
+[anchr]key_struct_a_2
+[dist4]key_struct_a_2_member  // num_bytes
+[u4]0                         // version
+[u8]5678                      // i
+[anchr]key_struct_a_2_member
+
+[anchr]value_array_ptr
+[dist4]value_array_member  // num_bytes
+[u4]2                      // num_elements
+[u1]1
+[u1]2
+[anchr]value_array_member
+
+[u4]0 [u2]0  // manual padding for array alignment