[vm] Fix flaky crash in StackTraceMallocHookLengthTest.

runtime/vm/malloc_hooks_test.cc

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
 
 #if defined(DART_USE_TCMALLOC) && !defined(PRODUCT) && !defined(TARGET_ARCH_DBC)
 
 #include "platform/assert.h"
 #include "vm/globals.h"
@@ skipping 12 matching lines @@
   for (uintptr_t i = 0; i < size; ++i) {
     buffer[i] = i;
   }
 }
 
 class EnableMallocHooksScope : public ValueObject {
  public:
   EnableMallocHooksScope() {
     saved_enable_malloc_hooks_ = FLAG_profiler_native_memory;
     FLAG_profiler_native_memory = true;
-    Profiler::InitAllocationSampleBuffer();
     MallocHooks::InitOnce();
     MallocHooks::ResetStats();
   }
 
   ~EnableMallocHooksScope() {
     MallocHooks::TearDown();
     FLAG_profiler_native_memory = saved_enable_malloc_hooks_;
   }
 
  private:
   bool saved_enable_malloc_hooks_;
 };
 
 class EnableMallocHooksAndStacksScope : public EnableMallocHooksScope {
  public:
   EnableMallocHooksAndStacksScope() {
     saved_enable_stack_traces_ = MallocHooks::stack_trace_collection_enabled();
     MallocHooks::set_stack_trace_collection_enabled(true);
     if (!FLAG_profiler) {
       FLAG_profiler = true;
       Profiler::InitOnce();
     }
+    MallocHooks::ResetStats();
   }
 
   ~EnableMallocHooksAndStacksScope() {
     MallocHooks::set_stack_trace_collection_enabled(saved_enable_stack_traces_);
   }
 
  private:
   bool saved_enable_stack_traces_;
 };
 
@@ skipping 124 matching lines @@
   EXPECT_SUBSTRING("\"dart::TestCase::Run()\"", json);
   EXPECT_SUBSTRING("\"dart::TestCaseBase::RunTest()\"", json);
   EXPECT_SUBSTRING("\"main\"", json);
 
   free(var);
 }
 
 };  // namespace dart
 
 #endif  // defined(DART_USE_TCMALLOC) && !defined(PRODUCT)