Roll libwebp to v0.4.2 (latest stable) to fix annoying build warning.

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

[mtklein, 2014-12-15 20:00:06]

mtklein@google.com changed reviewers:
+ caryclark@google.com, mtklein@google.com

[mtklein, 2014-12-15 20:00:14]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-12-15 20:00:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/807553002/1

[commit-bot: I haz the power, 2014-12-15 20:00:43]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2014-12-16 02:00 UTC

[caryclark, 2014-12-15 20:10:08]

lgtm

[commit-bot: I haz the power, 2014-12-15 20:11:53]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/e655557a551c545f4153b0764269298cd01cd0c0

[mtklein, 2014-12-15 20:22:22]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/795823005/ by mtklein@google.com.

The reason for reverting is: uses __builtins not available on all our compilers.