Realize IWYU pattern for handles.h header.

src/string-stream.h

Index: src/string-stream.h
diff --git a/src/string-stream.h b/src/string-stream.h
index 7aa5fd7db3d3e293e4d20a4c3a514f53dcf20615..d03f1b04c9c89b86583aceddc2a8017ba1c0555b 100644
--- a/src/string-stream.h
+++ b/src/string-stream.h
@@ -5,11 +5,17 @@
 #ifndef V8_STRING_STREAM_H_
 #define V8_STRING_STREAM_H_
 
+#include "src/allocation.h"
+#include "src/base/smart-pointers.h"
 #include "src/handles.h"
+#include "src/vector.h"
 
 namespace v8 {
 namespace internal {
 
+// Forward declarations.
+class ByteArray;
+
 class StringAllocator {
  public:
   virtual ~StringAllocator() { }