[turbofan] Remove --strict-heap-broker flag.

This has been on by default and the flag itself is no
longer useful.

R=jarin@chromium.org, mslekova@chromium.org

Bug: v8:7790
Change-Id: Icdf111b974a01953ea775ccb96d50217f3c8321b
Reviewed-on: https://chromium-review.googlesource.com/c/1342918
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57637}

src/compiler/js-heap-broker.cc

@@ -1512,10 +1512,7 @@ void JSHeapBroker::Retire() {
   mode_ = kRetired;
 }
 
-bool JSHeapBroker::SerializingAllowed() const {
-  return mode() == kSerializing ||
-         (!FLAG_strict_heap_broker && mode() == kSerialized);
-}
+bool JSHeapBroker::SerializingAllowed() const { return mode() == kSerializing; }
 
 void JSHeapBroker::SetNativeContextRef() {
   native_context_ = NativeContextRef(this, isolate()->native_context());
@@ -2350,8 +2347,7 @@ ObjectRef::ObjectRef(JSHeapBroker* broker, Handle<Object> object)
     : broker_(broker) {
   switch (broker->mode()) {
     case JSHeapBroker::kSerialized:
-      data_ = FLAG_strict_heap_broker ? broker->GetData(object)
-                                      : broker->GetOrCreateData(object);
+      data_ = broker->GetData(object);
       break;
     case JSHeapBroker::kSerializing:
       data_ = broker->GetOrCreateData(object);

src/flag-definitions.h

@@ -391,7 +391,6 @@ DEFINE_BOOL(block_concurrent_recompilation, false,
 DEFINE_BOOL(concurrent_inlining, false,
             "run optimizing compiler's inlining phase on a separate thread")
 DEFINE_IMPLICATION(future, concurrent_inlining)
-DEFINE_BOOL(strict_heap_broker, true, "fail on incomplete serialization")
 DEFINE_BOOL(trace_heap_broker, false, "trace the heap broker")
 
 // Flags for stress-testing the compiler.