[arm-simulator] Dispose stray Isolate in test

Bug: 
Change-Id: I6b853f9462793c14aca3cc45c735ddcef6ed6155
Reviewed-on: https://chromium-review.googlesource.com/704637
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48339}

test/cctest/test-simulator-arm.cc

@@ -279,15 +279,16 @@ class MemoryAccessThread : public v8::base::Thread {
         test_data_(NULL),
         is_finished_(false),
         has_request_(false),
-        did_request_(false) {}
+        did_request_(false),
+        isolate_(nullptr) {}
 
   virtual void Run() {
     v8::Isolate::CreateParams create_params;
     create_params.array_buffer_allocator = CcTest::array_buffer_allocator();
-    v8::Isolate* isolate = v8::Isolate::New(create_params);
-    Isolate* i_isolate = reinterpret_cast<Isolate*>(isolate);
-    v8::Isolate::Scope scope(isolate);
-
+    isolate_ = v8::Isolate::New(create_params);
+    Isolate* i_isolate = reinterpret_cast<Isolate*>(isolate_);
+    {
+      v8::Isolate::Scope scope(isolate_);
       v8::base::LockGuard<v8::base::Mutex> lock_guard(&mutex_);
       while (!is_finished_) {
         while (!(has_request_ || is_finished_)) {
@@ -304,6 +305,8 @@ class MemoryAccessThread : public v8::base::Thread {
         did_request_cv_.NotifyOne();
       }
     }
+    isolate_->Dispose();
+  }
 
   void NextAndWait(TestData* test_data, MemoryAccess access) {
     DCHECK(!has_request_);
@@ -333,6 +336,7 @@ class MemoryAccessThread : public v8::base::Thread {
   v8::base::Mutex mutex_;
   v8::base::ConditionVariable has_request_cv_;
   v8::base::ConditionVariable did_request_cv_;
+  v8::Isolate* isolate_;
 };
 
 TEST(simulator_invalidate_exclusive_access_threaded) {