Use new SkBaseMutex::assertHeld() in DiscardableMemoryPool

src/lazy/SkDiscardableMemoryPool.cpp

Index: src/lazy/SkDiscardableMemoryPool.cpp
diff --git a/src/lazy/SkDiscardableMemoryPool.cpp b/src/lazy/SkDiscardableMemoryPool.cpp
index 1d63a0bad710985b7dc2ab127554e77906075753..db2754e473ae37a736ac23a6a5d2957be10e8238 100644
--- a/src/lazy/SkDiscardableMemoryPool.cpp
+++ b/src/lazy/SkDiscardableMemoryPool.cpp
@@ -145,9 +145,9 @@ DiscardableMemoryPool::~DiscardableMemoryPool() {
 }
 
 void DiscardableMemoryPool::dumpDownTo(size_t budget) {
-    // assert((NULL = fMutex) || fMutex->isLocked());
-    // TODO(halcanary) implement bool fMutex::isLocked().
-    // WARNING: only call this function after aquiring lock.
+    if (fMutex != NULL) {
+        fMutex->assertHeld();
+    }
     if (fUsed <= budget) {
         return;
     }