SkDiscardableMemoryPool: always have a mutex for testing

Change-Id: I898058f1125f62832f2b2b661c661efb0f84da6d
Reviewed-on: https://skia-review.googlesource.com/14274
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Herb Derby <herb@google.com>
diff --git a/tests/ImageCacheTest.cpp b/tests/ImageCacheTest.cpp
index 9472388..457df91 100644
--- a/tests/ImageCacheTest.cpp
+++ b/tests/ImageCacheTest.cpp
@@ -121,7 +121,7 @@
         test_cache(reporter, cache, true);
     }
     {
-        sk_sp<SkDiscardableMemoryPool> pool(SkDiscardableMemoryPool::Make(defLimit, nullptr));
+        sk_sp<SkDiscardableMemoryPool> pool(SkDiscardableMemoryPool::Make(defLimit));
         gPool = pool.get();
         SkResourceCache cache(pool_factory);
         test_cache(reporter, cache, true);