SQM Fuzzer: Invalid vptr in base::TestMockTimeTaskRunner.

Bad-cast to base::TestMockTimeTaskRunner from invalid vptr
in base::TestMockTimeTaskRunner::MockClock::NowTicks

test_task_runner_ was getting destructed before the sequence
manager (which calls the test_task_runner_ upon its
destruction).

Bug: 867790
Change-Id: I412b6a720b4607deeaad1ce80d8bd601dfc04814
Reviewed-on: https://chromium-review.googlesource.com/1152979
Commit-Queue: Farah Charab <farahcharab@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578662}

third_party/blink/renderer/platform/scheduler/base/sequence_manager_fuzzer_processor.h

@@ -131,12 +131,12 @@ class PLATFORM_EXPORT SequenceManagerFuzzerProcessor {
                            ActionForTest::ActionType type,
                            TimeTicks start_time);
 
-  std::unique_ptr<SequenceManagerForTest> manager_;
-
   // Bound to current thread. Used to control the clock of the task queue
   // manager.
   scoped_refptr<TestMockTimeTaskRunner> test_task_runner_;
 
+  std::unique_ptr<SequenceManagerForTest> manager_;
+
   // For testing purposes, this should follow the order in which the queues were
   // created.
   std::vector<TaskQueueWithVoters> task_queues_;