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/SkResourceCacheTest.cpp b/tests/SkResourceCacheTest.cpp
index 7a9c876..5f8770d 100644
--- a/tests/SkResourceCacheTest.cpp
+++ b/tests/SkResourceCacheTest.cpp
@@ -127,7 +127,7 @@
         testBitmapCache_discarded_bitmap(reporter, &cache, nullptr);
     }
     {
-        sk_sp<SkDiscardableMemoryPool> pool(SkDiscardableMemoryPool::Make(byteLimit, nullptr));
+        sk_sp<SkDiscardableMemoryPool> pool(SkDiscardableMemoryPool::Make(byteLimit));
         gPool = pool.get();
         SkResourceCache::DiscardableFactory factory = pool_factory;
         SkResourceCache cache(factory);