James Price a84ebc3af9 tint: Add forward declaration for CastableBase
This fixes the google3 build, which compiles headers in isolation.

Change-Id: I3c9a91caa75c6da1d9dcc17bb6d2aeb5834c197e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/115440
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Auto-Submit: James Price <jrprice@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
2022-12-22 14:32:16 +00:00
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