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/DiscardableMemoryPoolTest.cpp b/tests/DiscardableMemoryPoolTest.cpp
index b8fd6d7..c91e827 100644
--- a/tests/DiscardableMemoryPoolTest.cpp
+++ b/tests/DiscardableMemoryPoolTest.cpp
@@ -9,8 +9,7 @@
 #include "Test.h"
 
 DEF_TEST(DiscardableMemoryPool, reporter) {
-    sk_sp<SkDiscardableMemoryPool> pool(
-        SkDiscardableMemoryPool::Make(1, nullptr));
+    sk_sp<SkDiscardableMemoryPool> pool(SkDiscardableMemoryPool::Make(1));
     pool->setRAMBudget(3);
     REPORTER_ASSERT(reporter, 0 == pool->getRAMUsed());