Add override for overridden functions under /gin

This CL was uploaded by git cl split.

R=jochen@chromium.org

Bug: 428099
Change-Id: Ibf3ee895938492cad26720d8b1d00a9fb102b033
Reviewed-on: https://chromium-review.googlesource.com/1032466
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554402}

gin/function_template.h

@@ -77,7 +77,7 @@ class CallbackHolder : public CallbackHolderBase {
   InvokerOptions invoker_options;
 
  private:
-  virtual ~CallbackHolder() {}
+  ~CallbackHolder() override {}
 
   DISALLOW_COPY_AND_ASSIGN(CallbackHolder);
 };

gin/wrappable.h

@@ -100,7 +100,7 @@ class Wrappable : public WrappableBase {
 
  protected:
   Wrappable() {}
-  virtual ~Wrappable() {}
+  ~Wrappable() override {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Wrappable);