Update libwebp to v0.5.2-rc2

This matches the version used by Chromium (crbug.com/675306) and
Android (https://android-review.googlesource.com/#/c/316352/).

Update our test. In the new version, we successfully decode a subset
from incomplete input that we previously did not.

Change-Id: I3442bf59ffdf223a723d8aa75f5b9b816b9e9c3c
Reviewed-on: https://skia-review.googlesource.com/6320
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
2 files changed