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() {
+}

generator/templates/mock_api.h

@@ -90,6 +90,8 @@ class ProcTableAsClass {
 
 class MockProcTable : public ProcTableAsClass {
     public:
+        MockProcTable();
+
         {% for type in by_category["object"] %}
             {% for method in type.methods if len(method.arguments) < 10 %}
                 MOCK_METHOD{{len(method.arguments) + 1}}(