Remove grab-bag includes of v8.h from everywhere.

src/snapshot/snapshot.h

Index: src/snapshot/snapshot.h
diff --git a/src/snapshot/snapshot.h b/src/snapshot/snapshot.h
index 7f89213f16c41c56d881cac7d7ba14ee15a59b2c..dabc694d50a7c01ecc13774f2b9a32c7751f5260 100644
--- a/src/snapshot/snapshot.h
+++ b/src/snapshot/snapshot.h
@@ -2,15 +2,20 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "src/isolate.h"
-#include "src/snapshot/serialize.h"
-
 #ifndef V8_SNAPSHOT_SNAPSHOT_H_
 #define V8_SNAPSHOT_SNAPSHOT_H_
 
+#include "src/isolate.h"
+#include "src/snapshot/serialize.h"
+
 namespace v8 {
 namespace internal {
 
+// Forward declarations.
+class Isolate;
+class PartialSerializer;
+class StartupSerializer;
+
 class Snapshot : public AllStatic {
  public:
   class Metadata {