Chinmay Garde c243987f16 Fix race in SkiaGPUObject unit-tests. (flutter/engine#16351)
There are two issues in the test as written:
* There is a race on the first check to dtor_task_queue_id which might be
  encountered if the calling thread is de-scheduled and the unref queue manages
  to collect the object before the end of the scope.
* Two threads were owning a shared object but we relied on the object to be
  collected on the unref queue.
2020-02-03 19:57:43 -08:00
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