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/pdf/SkPDFDevice.h b/src/pdf/SkPDFDevice.h
index 303d450..93eb78d 100644
--- a/src/pdf/SkPDFDevice.h
+++ b/src/pdf/SkPDFDevice.h
@@ -72,7 +72,7 @@
         return new SkPDFDevice(pageSize, rasterDpi, doc, false);
     }
 
-    virtual ~SkPDFDevice();
+    ~SkPDFDevice() override;
 
     /** These are called inside the per-device-layer loop for each draw call.
      When these are called, we have already applied any saveLayer operations,