[iwyu] Include handles.h less.

src/futex-emulation.h

Index: src/futex-emulation.h
diff --git a/src/futex-emulation.h b/src/futex-emulation.h
index a0e2b18bdc2cc3d24c966a8346f57e62e105f945..f1f3655dc74f49f26331840e4fffeeb971ee289d 100644
--- a/src/futex-emulation.h
+++ b/src/futex-emulation.h
@@ -13,7 +13,6 @@
 #include "src/base/macros.h"
 #include "src/base/platform/condition-variable.h"
 #include "src/base/platform/mutex.h"
-#include "src/handles.h"
 
 // Support for emulating futexes, a low-level synchronization primitive. They
 // are natively supported by Linux, but must be emulated for other platforms.
@@ -31,6 +30,8 @@ class TimeDelta;
 
 namespace internal {
 
+template <typename T>
+class Handle;
 class Isolate;
 class JSArrayBuffer;