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..5f18736ba64eb7710aea527883d19a177d4d5dc2
--- /dev/null
+++ b/mojo/public/interfaces/bindings/tests/data/validation/conformance_mthd20_good.data
@@ -0,0 +1,48 @@
+[dist4]message_header  // num_bytes
+[u4]0                  // version
+[u4]0                  // interface ID
+[u4]10                 // name

[yzshen1, 2016/09/20 23:44:40]

This should be 20. Otherwise we are not testing the right method.

[tibell, 2016/09/21 07:10:53]

Done.

+[u4]0                  // flags
+[u4]0                  // padding
+[anchr]message_header
+
+[dist4]method10_params  // num_bytes

[yzshen1, 2016/09/20 23:44:40]

please update the name accordingly. This is method 20.

[tibell, 2016/09/21 07:10:53]

Done.

+[u4]0                   // version
+[dist8]map_data_ptr     // param0
+[anchr]method10_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

[yzshen1, 2016/09/20 23:44:40]

I don't think this test case matches the parameter definition of Method20.

[tibell, 2016/09/21 07:10:53]

Done.

I changed my mind for the map value type half way through the CL and forgot to
update the mojom.

+[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
+
+[u4]0 [u1]0 [u1]0 [u1]0 [u1]0  // manual padding for array alignment

[tibell, 2016/09/21 07:10:53]

Something is wrong with the alignment (in the updated version I'm about to
post). I can't figure it out. Each struct/array starts at a 8 byte alignment.
Can you see the error?

+
+[anchr]key_struct_a_2
+[dist4]key_struct_a_2_member  // num_bytes
+[u4]0                         // version
+[u8]1234                      // i
+[anchr]key_struct_a_2_member
+
+[u4]0 [u1]0 [u1]0 [u1]0 [u1]0  // manual padding for array alignment
+
+[anchr]value_array_ptr
+[dist4]value_array_member  // num_bytes
+[u4]2                      // num_elements
+1 2
+[anchr]value_array_member