Wire: Add support for nullptr objects as arguments

BUG=dawn:5

Change-Id: I84c80796e9bdfbcf59fafdd38d33ab2101a1d795
Reviewed-on: https://dawn-review.googlesource.com/1541
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

generator/templates/dawn_wire/WireClient.cpp

@@ -215,6 +215,9 @@ namespace dawn_wire {
                 // Implementation of the ObjectIdProvider interface
                 {% for type in by_category["object"] %}
                     ObjectId GetId({{as_cType(type.name)}} object) const override {
+                        if (object == nullptr) {
+                            return 0;
+                        }
                         return reinterpret_cast<{{as_wireType(type)}}>(object)->id;
                     }
                 {% endfor %}

generator/templates/dawn_wire/WireServer.cpp

@@ -358,6 +358,11 @@ namespace dawn_wire {
                 // Implementation of the ObjectIdResolver interface
                 {% for type in by_category["object"] %}
                     DeserializeResult GetFromId(ObjectId id, {{as_cType(type.name)}}* out) const override {
+                        if (id == 0) {
+                            *out = nullptr;
+                            return DeserializeResult::Success;
+                        }
+
                         auto data = mKnown{{type.name.CamelCase()}}.Get(id);
                         if (data == nullptr) {
                             return DeserializeResult::FatalError;