[flags] Enable freezing of flags

This enables the --freeze-flags-after-init flag globally. Note that
tests, fuzzers, Node and other still explicitly disable the flag. The
chrome renderer process and default d8 execution will have it enabled
though.

R=cbruni@chromium.org

Bug: v8:12887
Change-Id: I9a15ef64227e5e6e04779d8d671a2c50d99c9097
Cq-Include-Trybots: luci.v8.try:v8_linux_blink_rel
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3695264Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81214}

src/flags/flag-definitions.h

@@ -1877,7 +1877,7 @@ DEFINE_BOOL(
     "Fuzzers use this flag to signal that they are ... fuzzing. This causes "
     "intrinsics to fail silently (e.g. return undefined) on invalid usage.")
 
-DEFINE_BOOL(freeze_flags_after_init, false,
+DEFINE_BOOL(freeze_flags_after_init, true,
             "Disallow changes to flag values after initializing V8")
 
 // mksnapshot.cc