Revert "[flags] Add a sanity check for unchanged jitless flags"

This reverts commit 3a46c81c.

Reason for revert: Breaking roll (or rather, oh no, cast_shell is broken, need to fix that before relanding): https://ci.chromium.org/ui/p/chromium/builders/try/cast_shell_linux/1053410/overview

Original change's description:
> [flags] Add a sanity check for unchanged jitless flags
>
> V8 flags in general should not change in a process after the
> first Isolate has been initialized. --jitless and related flags
> especially sensitive to this, so we introduce a dedicated check
> just for them.
>
> Bug: chromium:1262676, v8:9019, v8:12366
> Change-Id: I239726889d236a3785c1fdc076fa21d1b8983c92
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3260508
> Commit-Queue: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#77759}

Bug: chromium:1262676, v8:9019, v8:12366
Change-Id: Ie47d183bfd68633c3d30a13a038219051c38eba0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3268734
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Owners-Override: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#77785}

src/execution/isolate.cc

@@ -3017,56 +3017,6 @@ v8::PageAllocator* Isolate::page_allocator() const {
   return isolate_allocator_->page_allocator();
 }
 
-namespace {
-
-// These flags are all related to --jitless and *must not change within the
-// same process*. See also: crbug.com/v8/9019.
-enum class CheckedFlag {
-  kInitialized = 1 << 0,  // Whether initial_checked_flag_values_ has been set.
-  kJitless = 1 << 1,
-  kRegexpInterpretAll = 1 << 2,
-  kExposeWasm = 1 << 3,
-  kOpt = 1 << 4,
-};
-using CheckedFlags = base::Flags<CheckedFlag>;
-DEFINE_OPERATORS_FOR_FLAGS(CheckedFlags)
-
-// The first isolate sets these, all following isolates read and verify against
-// initial values.
-std::atomic<uint32_t> initial_checked_flag_values_;
-
-void VerifyCheckedFlags() {
-  // TODO(all): While jitless flags are especially sensitive and cause
-  // hard-to-debug failures if misused, it may be good to check all V8 flags in
-  // a similar fashion - or at least have a systematic way of defining flags
-  // that must not change. For now, this ad-hoc check for jitless-related flags
-  // is intended to avoid trivial and accidental misuses.
-  CheckedFlags current = CheckedFlag::kInitialized;
-  if (FLAG_jitless) current |= CheckedFlag::kJitless;
-  if (FLAG_regexp_interpret_all) current |= CheckedFlag::kRegexpInterpretAll;
-#if V8_ENABLE_WEBASSEMBLY
-  if (FLAG_expose_wasm) current |= CheckedFlag::kExposeWasm;
-#endif
-  if (FLAG_opt) current |= CheckedFlag::kOpt;
-
-  uint32_t initial_flags = 0;
-  const bool is_initial_isolate =
-      initial_checked_flag_values_.compare_exchange_strong(
-          initial_flags, static_cast<uint32_t>(current),
-          std::memory_order_relaxed, std::memory_order_relaxed);
-
-  if (is_initial_isolate) return;  // Nothing to check.
-
-  if (static_cast<uint32_t>(current) != initial_flags) {
-    FATAL(
-        "An Isolate was spawned with different flag values than another "
-        "Isolate in the same process. Flags must remain unchanged within one "
-        "process.");
-  }
-}
-
-}  // namespace
-
 Isolate::Isolate(std::unique_ptr<i::IsolateAllocator> isolate_allocator,
                  bool is_shared)
     : isolate_data_(this, isolate_allocator->GetPtrComprCageBase()),
@@ -3092,8 +3042,6 @@ Isolate::Isolate(std::unique_ptr<i::IsolateAllocator> isolate_allocator,
   TRACE_ISOLATE(constructor);
   CheckIsolateLayout();
 
-  VerifyCheckedFlags();
-
   // ThreadManager is initialized early to support locking an isolate
   // before it is entered.
   thread_manager_ = new ThreadManager(this);