Remove forward declaration of absl::FormatSink

absl says that it does not officially support forward declaration
of its classes.

Fixed: 42241501
Change-Id: Ice26c5d7fafb8444768ea58ea792b48a57115c88
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/187700
Auto-Submit: Austin Eng <enga@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
diff --git a/src/dawn/native/ObjectBase.h b/src/dawn/native/ObjectBase.h
index 3c98bdb..62edb9f 100644
--- a/src/dawn/native/ObjectBase.h
+++ b/src/dawn/native/ObjectBase.h
@@ -31,16 +31,13 @@
 #include <mutex>
 #include <string>
 
+#include "absl/strings/str_format.h"
 #include "dawn/common/LinkedList.h"
 #include "dawn/common/MutexProtected.h"
 #include "dawn/common/Ref.h"
 #include "dawn/common/RefCounted.h"
 #include "dawn/native/Forward.h"
 
-namespace absl {
-class FormatSink;
-}
-
 namespace dawn::native {
 
 namespace detail {