Mojo TypeScript bindings: avoid iterating all structs/unions

Structs and unions don't contain interface member kinds, so we can
reduce the bindings generation time by not collecting referenced kinds
from these cases.

Change-Id: I8b4a114a97503e045040fd1c911b4e057866ee6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4295736
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1111083}

mojo/public/tools/bindings/generators/mojom_ts_generator.py

@@ -395,10 +395,6 @@ def __init__(self, name, alias):
         for method in interface.methods:
           referenced_kinds.extend(method.parameters or [])
           referenced_kinds.extend(method.response_parameters or [])
-      for struct in self.module.structs:
-        referenced_kinds.extend(struct.fields)
-      for union in self.module.unions:
-        referenced_kinds.extend(union.fields)
 
       # Determine whether Remote and/or PendingReceiver are referenced.
       imports = []