add one-channel output support to webp encoder

BUG=
R=halcanary@google.com

Author: humper@google.com

Review URL: https://codereview.chromium.org/249853003

git-svn-id: http://skia.googlecode.com/svn/trunk@14340 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed