Avoid hang in OncePtr test with --threads 1

Avoid hang in OncePtr test when using "dm --threads 1".

The test will hang the threads until sk_num_cores() threads have run
the code. This requires that sk_num_cores() threads to be run in
parallel, which the global thread pool will not do if the thread count
is smaller than sk_num_cores().

BUG=skia:

Review URL: https://codereview.chromium.org/1419593004
3 files changed