src,test: disable freezing V8 flags on initialization

Node.js still changes flags after initializationg; either because
tests need to set their own flags (which V8 tests also still allow),
or because it's explicitly requested via the "v8.setFlagsFromString"
method that Node.js provides.

PR-URL: #44741
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node.cc

@@ -741,6 +741,11 @@ static ExitCode InitializeNodeWithArgsInternal(
   // used in diagnostic reports.
   per_process::cli_options->cmdline = *argv;
 
+  // Node provides a "v8.setFlagsFromString" method to dynamically change flags.
+  // Hence do not freeze flags when initializing V8. In a browser setting, this
+  // is security relevant, for Node it's less important.
+  V8::SetFlagsFromString("--no-freeze-flags-after-init");
+
 #if defined(NODE_V8_OPTIONS)
   // Should come before the call to V8::SetFlagsFromCommandLine()
   // so the user can disable a flag --foo at run-time by passing

test/cctest/node_test_fixture.cc

@@ -24,6 +24,11 @@ void NodeTestEnvironment::SetUp() {
 #endif
   cppgc::InitializeProcess(
       NodeZeroIsolateTestFixture::platform->GetPageAllocator());
+
+  // Before initializing V8, disable the --freeze-flags-after-init flag, so
+  // individual tests can set their own flags.
+  v8::V8::SetFlagsFromString("--no-freeze-flags-after-init");
+
   v8::V8::Initialize();
 }