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:

Review URL: https://codereview.chromium.org/807553002
2 files changed