[vm/concurrency] Remove ASSERT from IsMutatorThread to address r/w race isolate_ in vm thread.

Running tsan build locally on IsolateJson identified concurrent access to isolate_ from NoActiveIsolateScope/CollectNewSpaceGarbage and IsMutatorThread/SafepointThreads/Scavenge.

Change-Id: I4c6f5c83a7f6ddbfee8ede59443720db62a48b6f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151023
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

Author: aam

runtime/vm/thread.cc

@@ -631,13 +631,6 @@ void Thread::DeferredMarkingStackAcquire() {
       isolate_group()->deferred_marking_stack()->PopEmptyBlock();
 }
 
-bool Thread::IsMutatorThread() const {
-  if (isolate_ != nullptr) {
-    ASSERT(is_mutator_thread_ == (isolate_->mutator_thread() == this));
-  }
-  return is_mutator_thread_;
-}
-
 bool Thread::CanCollectGarbage() const {
   // We grow the heap instead of triggering a garbage collection when a
   // thread is at a safepoint in the following situations :

runtime/vm/thread.h

@@ -401,7 +401,8 @@ class Thread : public ThreadState {
     return OFFSET_OF(Thread, field_table_values_);
   }
 
-  bool IsMutatorThread() const;
+  bool IsMutatorThread() const { return is_mutator_thread_; }
+
   bool CanCollectGarbage() const;
 
   // Offset of Dart TimelineStream object.