Extract non-IO part of mksnapshot into an API method.

src/snapshot.h

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/isolate.h"
 
 #ifndef V8_SNAPSHOT_H_
 #define V8_SNAPSHOT_H_
 
 namespace v8 {
 namespace internal {
 
-class Snapshot {
+class Snapshot : public AllStatic {
  public:
   // Initialize the Isolate from the internal snapshot. Returns false if no
   // snapshot could be found.
   static bool Initialize(Isolate* isolate);
   // Create a new context using the internal partial snapshot.
   static Handle<Context> NewContextFromSnapshot(Isolate* isolate);
 
-  static const Vector<const byte> StartupSnapshot();
-  static const Vector<const byte> ContextSnapshot();
   static bool HaveASnapshotToStartFrom();
 
+  // To be implemented by the snapshot source.
   static const v8::StartupData SnapshotBlob();
 
+  static v8::StartupData CreateSnapshotBlob(
+      const Vector<const byte> startup_data,
+      const Vector<const byte> context_data);
+
+  static Vector<const byte> ExtractStartupData(const v8::StartupData* data);
+  static Vector<const byte> ExtractContextData(const v8::StartupData* data);
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(Snapshot);
 };
 
 #ifdef V8_USE_EXTERNAL_STARTUP_DATA
 void SetSnapshotFromFile(StartupData* snapshot_blob);
 #endif
 
 } }  // namespace v8::internal
 
 #endif  // V8_SNAPSHOT_H_