Add missing SK_OVERRIDE

Tested by running on clang head + ubuntu 14.04:
GYP_DEFINES=”skia_gpu=0 skia_warnings_as_errors=1" tools/xsan_build thread dm BUILDTYPE=Release
out/Release/dm -v

BUG=skia:3386

Review URL: https://codereview.chromium.org/894833002
12 files changed