Allow SkIcoCodec to fail due to conversion

Even if the client chooses dimensions that were suggested by
SkIcoCodec, it is possible for the color conversion to be incorrect,
so it can still reach the exit case of kInvalidConversion.

For example, when running nanobench, we attempt to decode the image
to Alpha_8, which the codec does not support. This allows running
nanobench in debug mode without asserting.

BUG=skia:3418

Review URL: https://codereview.chromium.org/1389943002
1 file changed