Update libwebp to 0.6.0

Update libwebp to 0.6.0

Corresponds with Android change
https://android-review.googlesource.com/#/c/326439/

"this version includes encoder and performance improvements"

Update build file
* Many files have been renamed from
    src/<subdir>/<name>.c
      to
    src/<subdir>/<name>_<subdir>.c

* Build new files (*_msa.c, *_neon.c, predictor_enc.c)

This should fix issue 5876, which was caused by a compiler bug.
With the added NEON implementation, we will no longer trigger
the bug.

BUG=skia:5876

CQ_INCLUDE_TRYBOTS=skia.primary:Build-Mac-Clang-arm-Debug-iOS

Review-Url: https://codereview.chromium.org/2689283007