update WebP to a dev version of 0.3.1

This is necessary to pick up a NEON fix for the iOS build.

R=borenet@google.com

Review URL: https://codereview.chromium.org/16318003

git-svn-id: http://skia.googlecode.com/svn/trunk@9516 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed