Grab mutex before reseting vk command pools.
Grab mutex before reseting vk command pools.

We have another thread that may be adding pools back to the available
array the same time we are resetting it. Make sure we grab a mutex before
doing the reset.

Change-Id: I59c6acf7b654d326241f097fc995e17418d5c2e9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/390480
Reviewed-by: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>
Commit-Queue: Greg Daniel <39df0a804564ccb6cf75f18db79653821f37c1c5@google.com>

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