[dawn][emscripten] Fix script path in GN build

This maybe assumes `dawn_emscripten_dir` is set to something starting
with `//` (path relative to project root, I think). I don't know if
there is a better way to handle it.

No-Try: true # we do not test the build that this affects
Change-Id: I1429f48f68b211c651fe0535bfef39f7c2826a27
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/212316
Commit-Queue: Kai Ninomiya <[email protected]>
Reviewed-by: Loko Kung <[email protected]>

Author: kainino0x

src/emdawnwebgpu/BUILD.gn

@@ -76,8 +76,7 @@ if (dawn_emscripten_dir != "") {
         ":emdawnwebgpu_js_gen",  # for struct_info_webgpu.json
       ]
 
-      script = rebase_path(dawn_emscripten_dir, root_build_dir) +
-               "/tools/maint/gen_struct_info.py"
+      script = dawn_emscripten_dir + "/tools/maint/gen_struct_info.py"
       args = [
         "-q",  # quiet
         rebase_path(sources[0], root_build_dir),  # input file