src: generate default snapshot with --predictable

To improve determinism of snapshot generation, add --predictable
to the V8 flags used to initialize a process launched to generate
snapshot. Also add a kGeneratePredictableSnapshot flag
to ProcessInitializationFlags for this and moves the configuration
of these flags into node::InitializeOncePerProcess() so that
it can be shared by embedders.

src/node.cc

@@ -838,6 +838,12 @@ static ExitCode InitializeNodeWithArgsInternal(
   V8::SetFlagsFromString(NODE_V8_OPTIONS, sizeof(NODE_V8_OPTIONS) - 1);
 #endif
 
+  if (!!(flags & ProcessInitializationFlags::kGeneratePredictableSnapshot) ||
+      per_process::cli_options->per_isolate->build_snapshot) {
+    v8::V8::SetFlagsFromString("--predictable");
+    v8::V8::SetFlagsFromString("--random_seed=42");
+  }
+
   // Specify this explicitly to avoid being affected by V8 changes to the
   // default value.
   V8::SetFlagsFromString("--rehash-snapshot");

src/node.h

@@ -265,6 +265,8 @@ enum Flags : uint32_t {
   // cppgc::InitializeProcess() before creating a Node.js environment
   // and call cppgc::ShutdownProcess() before process shutdown.
   kNoInitializeCppgc = 1 << 13,
+  // Initialize the process for predictable snapshot generation.
+  kGeneratePredictableSnapshot = 1 << 14,
 
   // Emulate the behavior of InitializeNodeWithArgs() when passing
   // a flags argument to the InitializeOncePerProcess() replacement

tools/snapshot/node_mksnapshot.cc

@@ -50,8 +50,6 @@ int main(int argc, char* argv[]) {
   setvbuf(stderr, nullptr, _IONBF, 0);
 #endif  // _WIN32
 
-  v8::V8::SetFlagsFromString("--random_seed=42");
-  v8::V8::SetFlagsFromString("--harmony-import-assertions");
   return BuildSnapshot(argc, argv);
 }
 
@@ -65,7 +63,8 @@ int BuildSnapshot(int argc, char* argv[]) {
 
   std::unique_ptr<node::InitializationResult> result =
       node::InitializeOncePerProcess(
-          std::vector<std::string>(argv, argv + argc));
+          std::vector<std::string>(argv, argv + argc),
+          node::ProcessInitializationFlags::kGeneratePredictableSnapshot);
 
   CHECK(!result->early_return());
   CHECK_EQ(result->exit_code(), 0);