Generate multiple variants of webgpu.h header

Adds a "tag" system so that entries of dawn.json can be conditionally
emitted in different configurations. With a few more dawn.json changes,
this will enable generating the exact upstream header.

Bug: dawn:1080
Change-Id: I3506dadd485e31786578a3a64c3603c964c5354f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/62580
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

generator/templates/webgpu_cpp.cpp

@@ -11,7 +11,11 @@
 //* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 //* See the License for the specific language governing permissions and
 //* limitations under the License.
-#include "dawn/webgpu_cpp.h"
+{% if 'dawn' in enabled_tags %}
+    #include "dawn/webgpu_cpp.h"
+{% else %}
+    #include "webgpu/webgpu_cpp.h"
+{% endif %}
 
 namespace wgpu {
     {% for type in by_category["enum"] %}