Descriptorize Buffer

We still keep a dummy BufferBuilder object around so that it can be used
for the builder error callback tests of the wire.

Change-Id: If0c502bb8b62ee3ed61815e34e9b6ee6c03a65ef

generator/templates/mock_api.cpp

@@ -108,3 +108,6 @@ void ProcTableAsClass::CallMapWriteCallback(dawnBuffer buffer, dawnBufferMapAsyn
         return reinterpret_cast<{{as_cType(type.name)}}>(mObjects.back().get());
     }
 {% endfor %}
+
+MockProcTable::MockProcTable() {
+}