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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