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

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index b08607bee6a531a512e02dd5153603c9b9fdac32..27a58a49426cad6e81f2b4a620b64129b7959dcb 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -5361,6 +5361,13 @@ class V8_EXPORT V8 {
   static void SetSnapshotDataBlob(StartupData* startup_blob);
 
   /**
+   * Create a new isolate and context for the purpose of capturing a snapshot
+   * Returns { NULL, 0 } on failure.
+   * The caller owns the data array in the return value.
+   */
+  static StartupData CreateSnapshotDataBlob();
+
+  /**
    * Adds a message listener.
    *
    * The same message listener can be added more than once and in that