[snapshot] revisit snapshot API.

src/snapshot/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.
 
 #ifndef V8_SNAPSHOT_SNAPSHOT_H_
 #define V8_SNAPSHOT_SNAPSHOT_H_
 
 #include "src/snapshot/partial-serializer.h"
 #include "src/snapshot/startup-serializer.h"
 
@@ skipping 45 matching lines @@
   // 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 MaybeHandle<Context> NewContextFromSnapshot(
       Isolate* isolate, Handle<JSGlobalProxy> global_proxy,
       size_t context_index);
 
   static bool HaveASnapshotToStartFrom(Isolate* isolate);
 
+  static bool HasContextSnapshot(Isolate* isolate, size_t index);
+
   static bool EmbedsScript(Isolate* isolate);
 
   static uint32_t SizeOfFirstPage(Isolate* isolate, AllocationSpace space);
 
 
   // To be implemented by the snapshot source.
   static const v8::StartupData* DefaultSnapshotBlob();
 
   static v8::StartupData CreateSnapshotBlob(
       const SnapshotData* startup_snapshot,
@@ skipping 40 matching lines @@
 };
 
 #ifdef V8_USE_EXTERNAL_STARTUP_DATA
 void SetSnapshotFromFile(StartupData* snapshot_blob);
 #endif
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_SNAPSHOT_H_