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/include/core/SkStream.h b/include/core/SkStream.h
index e10aece..428f247 100644
--- a/include/core/SkStream.h
+++ b/include/core/SkStream.h
@@ -261,7 +261,7 @@
      */
     explicit SkFILEStream(FILE* file);
 
-    virtual ~SkFILEStream();
+    ~SkFILEStream() override;
 
     /** Returns true if the current path could be opened. */
     bool isValid() const { return fFILE != nullptr; }
@@ -357,7 +357,7 @@
 class SK_API SkFILEWStream : public SkWStream {
 public:
     SkFILEWStream(const char path[]);
-    virtual ~SkFILEWStream();
+    ~SkFILEWStream() override;
 
     /** Returns true if the current path could be opened.
     */
@@ -377,7 +377,7 @@
 class SK_API SkDynamicMemoryWStream : public SkWStream {
 public:
     SkDynamicMemoryWStream();
-    virtual ~SkDynamicMemoryWStream();
+    ~SkDynamicMemoryWStream() override;
 
     bool write(const void* buffer, size_t size) override;
     size_t bytesWritten() const override;