Update libwebp and resume testing scaled webp

libwebp has a fix for [1]. Update to the commit that contains the fix.

Update libwebp.gypi, corresponding to libwebp's latest makefile.

Turn back on DM testing for scaled webp, now that it should no longer
use uninitialized memory.

Fix a warning in config.h

[1] https://code.google.com/p/webp/issues/detail?id=254

BUG=skia:4038

Review URL: https://codereview.chromium.org/1280073002
4 files changed