Add nullable annotations to webgpu.h

Bug: dawn:22
Change-Id: Ie14072e91bab3b69b8d98f15bcbdeaa6679ffcd7
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/91240
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

dawn.json

@@ -1065,12 +1065,22 @@
             },
             {
                 "name": "create swap chain",
+                "tags": ["dawn"],
                 "returns": "swap chain",
                 "args": [
                     {"name": "surface", "type": "surface", "optional": true},
                     {"name": "descriptor", "type": "swap chain descriptor", "annotation": "const*"}
                 ]
             },
+            {
+                "name": "create swap chain",
+                "tags": ["upstream", "emscripten"],
+                "returns": "swap chain",
+                "args": [
+                    {"name": "surface", "type": "surface"},
+                    {"name": "descriptor", "type": "swap chain descriptor", "annotation": "const*"}
+                ]
+            },
             {
                 "name": "create texture",
                 "returns": "texture",

generator/templates/api.h

@@ -100,6 +100,7 @@ typedef struct {{c_prefix}}ChainedStructOut {
         {% endif %}
         {% for member in type.members %}
             {{as_annotated_cType(member)}};
+            {%- if member.optional %} // nullable{% endif %}{{""}}
         {% endfor %}
     } {{as_cType(type.name)}};
 
@@ -143,6 +144,7 @@ extern "C" {
             {{-as_cType(type.name)}} {{as_varName(type.name)}}
             {%- for arg in method.arguments -%}
                 , {{as_annotated_cType(arg)}}
+                {%- if arg.optional %} /* nullable */{% endif %}
             {%- endfor -%}
         );
     {% endfor %}
@@ -167,6 +169,7 @@ extern "C" {
             {{-as_cType(type.name)}} {{as_varName(type.name)}}
             {%- for arg in method.arguments -%}
                 , {{as_annotated_cType(arg)}}
+                {%- if arg.optional %} /* nullable */{% endif %}
             {%- endfor -%}
         );
     {% endfor %}