Enable strict-virtual-specifiers for iOS builds.

ui/gfx/image/image.cc

Index: ui/gfx/image/image.cc
diff --git a/ui/gfx/image/image.cc b/ui/gfx/image/image.cc
index 46c1a44abd00f695ee3e053607243ac9cdda6197..170c51465df3f38fa92ab1299a6aeac7845fcf14 100644
--- a/ui/gfx/image/image.cc
+++ b/ui/gfx/image/image.cc
@@ -288,22 +288,16 @@ class ImageRepCocoaTouch : public ImageRep {
     CHECK(image);
   }
 
-  virtual ~ImageRepCocoaTouch() {
+  ~ImageRepCocoaTouch() override {
     base::mac::NSObjectRelease(image_);
     image_ = nil;
   }
 
-  virtual int Width() const override {
-    return Size().width();
-  }
+  int Width() const override { return Size().width(); }
 
-  virtual int Height() const override {
-    return Size().height();
-  }
+  int Height() const override { return Size().height(); }
 
-  virtual gfx::Size Size() const override {
-    return internal::UIImageSize(image_);
-  }
+  gfx::Size Size() const override { return internal::UIImageSize(image_); }
 
   UIImage* image() const { return image_; }