[flags] Make --young-generation-large-objects readonly

This flag is already baked into the snapshot by enabling more
write-barrier elimination, so changing it at runtime would be a bug.

Change-Id: I3bc73f3c880285ec46b69b0c44934f64b49912ee
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2290856
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69046}

src/flags/flag-definitions.h

@@ -1092,9 +1092,10 @@ DEFINE_BOOL(fast_promotion_new_space, false,
 
 DEFINE_BOOL(clear_free_memory, false, "initialize free memory with 0")
 
-DEFINE_BOOL(young_generation_large_objects, true,
-            "allocates large objects by default in the young generation large "
-            "object space")
+DEFINE_BOOL_READONLY(
+    young_generation_large_objects, true,
+    "allocates large objects by default in the young generation large "
+    "object space")
 
 // assembler-ia32.cc / assembler-arm.cc / assembler-arm64.cc / assembler-x64.cc
 DEFINE_BOOL(debug_code, DEBUG_BOOL,

test/cctest/heap/test-heap.cc

@@ -6057,7 +6057,7 @@ TEST(Regress618958) {
 
 TEST(YoungGenerationLargeObjectAllocationScavenge) {
   if (FLAG_minor_mc) return;
-  FLAG_young_generation_large_objects = true;
+  if (!FLAG_young_generation_large_objects) return;
   CcTest::InitializeVM();
   v8::HandleScope scope(CcTest::isolate());
   Heap* heap = CcTest::heap();
@@ -6087,7 +6087,7 @@ TEST(YoungGenerationLargeObjectAllocationScavenge) {
 
 TEST(YoungGenerationLargeObjectAllocationMarkCompact) {
   if (FLAG_minor_mc) return;
-  FLAG_young_generation_large_objects = true;
+  if (!FLAG_young_generation_large_objects) return;
   CcTest::InitializeVM();
   v8::HandleScope scope(CcTest::isolate());
   Heap* heap = CcTest::heap();
@@ -6117,7 +6117,7 @@ TEST(YoungGenerationLargeObjectAllocationMarkCompact) {
 
 TEST(YoungGenerationLargeObjectAllocationReleaseScavenger) {
   if (FLAG_minor_mc) return;
-  FLAG_young_generation_large_objects = true;
+  if (!FLAG_young_generation_large_objects) return;
   CcTest::InitializeVM();
   v8::HandleScope scope(CcTest::isolate());
   Heap* heap = CcTest::heap();