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

src/snapshot-external.cc

Index: src/snapshot-external.cc
diff --git a/src/snapshot-external.cc b/src/snapshot-external.cc
index 6a89d42a8ece8cd476ac2f8c0bb239d4429c3996..2fda57125220085a12ff0bf78224ab5d7ff72ec5 100644
--- a/src/snapshot-external.cc
+++ b/src/snapshot-external.cc
@@ -26,11 +26,10 @@ void SetSnapshotFromFile(StartupData* snapshot_blob) {
   DCHECK(snapshot_blob->data);
   DCHECK(snapshot_blob->raw_size > 0);
   DCHECK(!external_startup_blob.data);
-  external_startup_blob = *snapshot_blob;
-
   // Validate snapshot blob.
-  DCHECK(!Snapshot::StartupSnapshot().is_empty());
-  DCHECK(!Snapshot::ContextSnapshot().is_empty());
+  DCHECK(!Snapshot::ExtractStartupData(snapshot_blob).is_empty());
+  DCHECK(!Snapshot::ExtractContextData(snapshot_blob).is_empty());
+  external_startup_blob = *snapshot_blob;
 }