Revert of Roll libwebp to v0.4.2 (latest stable) to fix annoying build warning. (patchset #1 id:1 of https://codereview.chromium.org/807553002/)

Reason for revert:
uses __builtins not available on all our compilers

Original issue's description:
> Roll libwebp to v0.4.2 (latest stable) to fix annoying build warning.
>
> This warning should now go away:
>
> ../../third_party/externals/libwebp/src/enc/quant.c:105:23: warning: unused variable 'kCoeffThresh' [-Wunused-const-variable]
> static const uint16_t kCoeffThresh[16] = {
>
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/e655557a551c545f4153b0764269298cd01cd0c0

TBR=caryclark@google.com,mtklein@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/795823005
diff --git a/DEPS b/DEPS
index 41feed9..02b9182 100644
--- a/DEPS
+++ b/DEPS
@@ -15,7 +15,7 @@
   "third_party/externals/harfbuzz": "https://skia.googlesource.com/third_party/harfbuzz.git@0.9.35",
   "third_party/externals/jsoncpp" : "https://chromium.googlesource.com/external/jsoncpp/jsoncpp.git@1afff032c83e26ddf7f2776e8b43de5ad666c1fa",
   "third_party/externals/libjpeg" : "https://chromium.googlesource.com/chromium/deps/libjpeg_turbo.git@82ce8a6d4ebe12a177c0c3597192f2b4f09e81c3",
-  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@v0.4.2",
+  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@3fe91635df8734b23f3c1b9d1f0c4fa8cfaf4e39",
   "third_party/externals/nanomsg": "https://skia.googlesource.com/third_party/nanomsg.git@0.4-beta",
 
   "platform_tools/android/third_party/externals/expat" : "https://android.googlesource.com/platform/external/expat.git@android-4.2.2_r1.2",