Use palette only if we can in entropy analysis.
Use palette only if we can in entropy analysis.

Change-Id: I2be74b4d5fe48e059d5ced619d76fd030db79f74

PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp