link libwebpmux in system-webp builds

We use several symbols from this library... we currently fail to link.

(The other small changes in here are automatic from gn format.)

BUG=skia:6891

Change-Id: Iec6f5deceecdb61571827ebb502a9f7e7e4a4bef
Reviewed-on: https://skia-review.googlesource.com/29260
Commit-Queue: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Heather Miller <hcm@google.com>
1 file changed