[snapshot] Add flag to skip checksumming

For very large snapshots, calculating the checksum of the snapshot can
take at least 30% of the time to deserialize the snapshot, so add a flag
to skip it.

Change-Id: I8f3418f1d1e9f984e7334ac51096a1a166874722
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3001176
Auto-Submit: Dan Elphick <delphick@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75548}

src/flags/flag-definitions.h

@@ -1669,6 +1669,8 @@ DEFINE_BOOL(rcs_cpu_time, false,
 DEFINE_IMPLICATION(rcs_cpu_time, rcs)
 
 // snapshot-common.cc
+DEFINE_BOOL(skip_snapshot_checksum, false,
+            "Skip snapshot checksum calculation when deserializing an Isolate.")
 DEFINE_BOOL(profile_deserialization, false,
             "Print the time it takes to deserialize the snapshot.")
 DEFINE_BOOL(serialization_statistics, false,

src/snapshot/snapshot.cc

@@ -154,7 +154,9 @@ bool Snapshot::Initialize(Isolate* isolate) {
 
   const v8::StartupData* blob = isolate->snapshot_blob();
   SnapshotImpl::CheckVersion(blob);
-  CHECK(VerifyChecksum(blob));
+  if (!FLAG_skip_snapshot_checksum) {
+    CHECK(VerifyChecksum(blob));
+  }
   base::Vector<const byte> startup_data =
       SnapshotImpl::ExtractStartupData(blob);
   base::Vector<const byte> read_only_data =