dawn.h: Don't use anonymous structures

Instead make them have the same name as their typedef. This is because
anonymous structures is considered bad practice in C because it prevents
forward declaring them.

BUG=

Change-Id: I2d7a788a0d807a2689567d0bb220adaf5335e07a
Reviewed-on: https://dawn-review.googlesource.com/1521
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

generator/templates/api.h

@@ -36,7 +36,7 @@
 {% endfor %}
 
 {% for type in by_category["structure"] %}
-    typedef struct {
+    typedef struct {{as_cType(type.name)}} {
         {% if type.extensible %}
             const void* nextInChain;
         {% endif %}
@@ -44,6 +44,7 @@
             {{as_annotated_cType(member)}};
         {% endfor %}
     } {{as_cType(type.name)}};
+
 {% endfor %}
 
 // Custom types depending on the target language