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/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 79023a0..6b9a888 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -205,7 +205,7 @@
         fBackend = kNone_BackEndType;
     }
 
-    virtual ~DefaultDeviceManager() {
+    ~DefaultDeviceManager() override {
 #if SK_SUPPORT_GPU
         SkSafeUnref(fCurContext);
         SkSafeUnref(fCurIntf);