[test]: Fix platform lifetime in EagerUnmappingInCollectAllAvailableGarbage.

Currently MockPlatformForUnmapper has shorter lifetime than the isolate that
uses it. This leads to use-after-free races in concurrent tasks that fetch
the mock platform just before it is freed.

This CL ensures that MockPlatformForUnmapper is valid throughout the whole
lifetime of the isolate

Change-Id: I94a658bf7eb70d924a19522dab09744f21782972
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2502809Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70826}

test/cctest/heap/test-unmapper.cc

@@ -50,14 +50,24 @@ class MockPlatformForUnmapper : public TestPlatform {
   v8::Platform* old_platform_;
 };
 
-TEST(EagerUnmappingInCollectAllAvailableGarbage) {
+UNINITIALIZED_TEST(EagerUnmappingInCollectAllAvailableGarbage) {
   FLAG_stress_concurrent_allocation = false;  // For SimulateFullSpace.
-  CcTest::InitializeVM();
   MockPlatformForUnmapper platform;
-  Heap* heap = CcTest::heap();
-  i::heap::SimulateFullSpace(heap->old_space());
-  CcTest::CollectAllAvailableGarbage();
-  CHECK_EQ(0, heap->memory_allocator()->unmapper()->NumberOfChunks());
+  v8::Isolate::CreateParams create_params;
+  create_params.array_buffer_allocator = CcTest::array_buffer_allocator();
+  v8::Isolate* isolate = v8::Isolate::New(create_params);
+
+  {
+    v8::HandleScope handle_scope(isolate);
+    v8::Local<v8::Context> context = CcTest::NewContext(isolate);
+    v8::Context::Scope context_scope(context);
+    Isolate* i_isolate = reinterpret_cast<i::Isolate*>(isolate);
+    Heap* heap = i_isolate->heap();
+    i::heap::SimulateFullSpace(heap->old_space());
+    CcTest::CollectAllAvailableGarbage(i_isolate);
+    CHECK_EQ(0, heap->memory_allocator()->unmapper()->NumberOfChunks());
+  }
+  isolate->Dispose();
 }
 
 }  // namespace heap