Add error directives for api.h/api_cpp.h #ifdef __EMSCRIPTEN__

This could avoid including dawn's version of webgpu headers
and confusion at runtime when build with emscripten.
Emscripten's system/include/webgpu headers should be used.

Bug: None
Change-Id: Ie8c6ebc23f28a56b1cb887ae7f11d3f738151aec
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/79740
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Shrek Shao <shrekshao@google.com>

generator/templates/api_cpp.h

diff --git a/generator/templates/api_cpp.h b/generator/templates/api_cpp.h
index d54dcc3..2a207cc 100644
--- a/generator/templates/api_cpp.h
+++ b/generator/templates/api_cpp.h
@@ -13,6 +13,11 @@
 //* limitations under the License.
 {% set API = metadata.api.upper() %}
 {% set api = API.lower() %}
+{% if 'dawn' not in enabled_tags %}
+    #ifdef __EMSCRIPTEN__
+    #error "Do not include this header. Emscripten already provides headers needed for {{metadata.api}}."
+    #endif
+{% endif %}
 #ifndef {{API}}_CPP_H_
 #define {{API}}_CPP_H_