Expand SkMessageBus to support different unique key types

Bug: skia:11728
Change-Id: I16fb8250fa5c04ce3fe369a50d0c61a0bee46811
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/383696
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/GrBackendTextureImageGenerator.cpp b/src/gpu/GrBackendTextureImageGenerator.cpp
index 4e08ecc..620e18c 100644
--- a/src/gpu/GrBackendTextureImageGenerator.cpp
+++ b/src/gpu/GrBackendTextureImageGenerator.cpp
@@ -37,7 +37,7 @@
     // Generator has been freed, and no one is borrowing the texture. Notify the original cache
     // that it can free the last ref, so it happens on the correct thread.
     GrTextureFreedMessage msg { fOriginalTexture, fOwningContextID };
-    SkMessageBus<GrTextureFreedMessage>::Post(msg);
+    SkMessageBus<GrTextureFreedMessage, uint32_t>::Post(msg);
 }
 
 std::unique_ptr<SkImageGenerator>