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