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/tools/gpu/gl/debug/GrBufferObj.h b/tools/gpu/gl/debug/GrBufferObj.h
index c91606d..d0217b2 100644
--- a/tools/gpu/gl/debug/GrBufferObj.h
+++ b/tools/gpu/gl/debug/GrBufferObj.h
@@ -25,7 +25,7 @@
         , fSize(0)
         , fUsage(GR_GL_STATIC_DRAW) {
     }
-    virtual ~GrBufferObj() {
+    ~GrBufferObj() override {
         delete[] fDataPtr;
     }