Mark SkGifImageReader destructors with override

Very new versions of Clang (5.0 TOT) call out this problem.

Bug: skia:
Change-Id: If90f658f258ca52ebb21e2352fbf3180c41008bd
Reviewed-on: https://skia-review.googlesource.com/17795
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>

third_party/gif/SkGifImageReader.h

@@ -212,7 +212,7 @@ public:
     {
     }
 
-    ~SkGIFFrameContext()
+    ~SkGIFFrameContext() override
     {
     }
 
@@ -293,7 +293,7 @@ public:
     {
     }
 
-    ~SkGifImageReader()
+    ~SkGifImageReader() override
     {
     }