SnapshotCreator: start from existing snapshot if we have one

src/snapshot/snapshot-common.cc

Index: src/snapshot/snapshot-common.cc
diff --git a/src/snapshot/snapshot-common.cc b/src/snapshot/snapshot-common.cc
index 1658b3bdcf307c561435eb1d01da2db4cca45678..a2fbdd5f2aa4d15e758fef87a31b5f8b961f165b 100644
--- a/src/snapshot/snapshot-common.cc
+++ b/src/snapshot/snapshot-common.cc
@@ -195,7 +195,7 @@ SnapshotData::SnapshotData(const Serializer* serializer) {
 
   // Set header values.
   SetMagicNumber(serializer->isolate());
-  SetHeaderValue(kCheckSumOffset, Version::Hash());
+  SetHeaderValue(kVersionHashOffset, Version::Hash());
   SetHeaderValue(kNumReservationsOffset, reservations.length());
   SetHeaderValue(kPayloadLengthOffset, payload->length());
 
@@ -209,7 +209,7 @@ SnapshotData::SnapshotData(const Serializer* serializer) {
 }
 
 bool SnapshotData::IsSane() {
-  return GetHeaderValue(kCheckSumOffset) == Version::Hash();
+  return GetHeaderValue(kVersionHashOffset) == Version::Hash();
 }
 
 Vector<const SerializedData::Reservation> SnapshotData::Reservations() const {