Fix up -Winconsistent-missing-override

gm/bitmapshader.cpp

Index: gm/bitmapshader.cpp
diff --git a/gm/bitmapshader.cpp b/gm/bitmapshader.cpp
index 1d866c601067d4a93443d11d393d79d20d08166f..3e664566164c2357ae80415d580227e2d7cddf2a 100644
--- a/gm/bitmapshader.cpp
+++ b/gm/bitmapshader.cpp
@@ -49,15 +49,15 @@ protected:
         draw_mask(&fMask);
     }
 
-    virtual SkString onShortName() {
+    SkString onShortName() override {
         return SkString("bitmapshaders");
     }
 
-    virtual SkISize onISize() {
+    SkISize onISize() override {
         return SkISize::Make(150, 100);
     }
 
-    virtual void onDraw(SkCanvas* canvas) {
+    void onDraw(SkCanvas* canvas) override {
         SkPaint paint;
 
         for (int i = 0; i < 2; i++) {