Use the upstream version of libwebp, v0.4.3.

third_party/libwebp/README.chromium

Index: third_party/libwebp/README.chromium
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+Name: WebP image encoder/decoder
+Short Name: libwebp
+URL: http://developers.google.com/speed/webp
+Version: v0.4.2
+License: BSD
+License File: LICENSE
+Security Critical: Yes
+
+Description:
+Source archive:
+  http://downloads.webmproject.org/releases/webp/libwebp-0.4.2.tar.gz
+
+WebP is an image format that does both lossy and lossless compression of
+digital photographic images. WebP consists of a codec based on VP8, that Google
+open-sourced in May 2010 and a container based on RIFF. Webmasters, web
+developers and browser developers can use WebP to compress, archive and
+distribute digital images more efficiently.
+
+Local changes:
+ * Removed examples/, documentation and build related files, keeping only
+   the contents of src/ less mux/ which is unused.
+ * Merged COPYING/PATENTS to LICENSE
+Cherry-picks:
+  Revert patch f7fc4bc: dec/webp.c: don't wait for data before reporting w/h
+  6638710 webp/types.h: use inline for clang++/-std=c++11
+  a3946b8 enc_neon: fix building with non-Xcode clang (iOS)
+  783a8cd Regression fix for lossless decoding