Cleanup: Mark some overridden methods with 'SK_OVERRIDE'.

This fixes errors like this:

../../include/gpu/effects/GrPorterDuffXferProcessor.h:27:25: error:
'name' overrides a member function but is not marked 'override'
[-Werror,-Winconsistent-missing-override]

BUG=skia:3075
TEST=ninja -C out/Debug skia_lib
TBR=bsalomon@google.com

Review URL: https://codereview.chromium.org/804813002
diff --git a/src/gpu/SkGpuDevice.h b/src/gpu/SkGpuDevice.h
index 9023a63..b600f99 100644
--- a/src/gpu/SkGpuDevice.h
+++ b/src/gpu/SkGpuDevice.h
@@ -95,7 +95,7 @@
                                 const SkPaint& paint,
                                 SkCanvas::DrawBitmapRectFlags flags) SK_OVERRIDE;
     virtual void drawSprite(const SkDraw&, const SkBitmap& bitmap,
-                            int x, int y, const SkPaint& paint);
+                            int x, int y, const SkPaint& paint) SK_OVERRIDE;
     virtual void drawText(const SkDraw&, const void* text, size_t len,
                           SkScalar x, SkScalar y, const SkPaint&) SK_OVERRIDE;
     virtual void drawPosText(const SkDraw&, const void* text, size_t len,