Mark overridden destructors with 'override' and remove 'virtual'

This silences a new warning in clang 5.0

Change-Id: Ieb5b75a6ffed60107c3fd16075d2ecfd515b55e8
Reviewed-on: https://skia-review.googlesource.com/10006
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/src/lazy/SkDiscardableMemoryPool.cpp b/src/lazy/SkDiscardableMemoryPool.cpp
index fece7f0..2cb1b3f 100644
--- a/src/lazy/SkDiscardableMemoryPool.cpp
+++ b/src/lazy/SkDiscardableMemoryPool.cpp
@@ -31,7 +31,7 @@
      *  Without mutex, will be not be thread safe.
      */
     DiscardableMemoryPool(size_t budget, SkBaseMutex* mutex = nullptr);
-    virtual ~DiscardableMemoryPool();
+    ~DiscardableMemoryPool() override;
 
     SkDiscardableMemory* create(size_t bytes) override;
 
@@ -80,7 +80,7 @@
 public:
     PoolDiscardableMemory(DiscardableMemoryPool* pool,
                             void* pointer, size_t bytes);
-    virtual ~PoolDiscardableMemory();
+    ~PoolDiscardableMemory() override;
     bool lock() override;
     void* data() override;
     void unlock() override;