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>
diff --git a/DEPS b/DEPS
index 7256fd0..60c7ce9 100644
--- a/DEPS
+++ b/DEPS
@@ -15,7 +15,7 @@
   "third_party/externals/gyp"     : "https://chromium.googlesource.com/external/gyp.git@87ac4d0e63fc7dd8152a350327fea8dcf031bf56",
   "third_party/externals/harfbuzz": "https://skia.googlesource.com/third_party/harfbuzz.git@1.3.0",
   "third_party/externals/jsoncpp" : "https://chromium.googlesource.com/external/github.com/open-source-parsers/jsoncpp.git@1.0.0",
-  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@v0.5.0",
+  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@v0.5.2-rc2",
   "third_party/externals/zlib"    : "https://chromium.googlesource.com/chromium/src/third_party/zlib@4576304a4b9835aa8646c9735b079e1d96858633",
 
   "third_party/externals/dng_sdk" : "https://android.googlesource.com/platform/external/dng_sdk.git@96443b262250c390b0caefbf3eed8463ba35ecae",