Fix build with system libwebp

This adds webpmux to linked libraries to fix the following compile error:

python "../../build/toolchain/gcc_link_wrapper.py" --output="./font_service.service" -- x86_64-pc-linux-gnu-g++ -fPIC -Wl,-z,noexecstack -Wl,-z,now -Wl,-z,relro -Wl,-z,defs -Wl,--no-as-needed -lpthread -Wl,--as-needed -m64 -pthread -Wl,-O1 -Wl,--gc-sections -Wl,-rpath-link=. -Wl,--disable-new-dtags -Wl,--export-dynamic -Wl,-O1 -Wl,--as-needed -o "./font_service.service" -Wl,--start-group @"./font_service.service.rsp"  -Wl,--end-group   -ldl -lrt -lgmodule-2.0 -lgobject-2.0 -lgthread-2.0 -lglib-2.0 -licui18n -licuuc -licudata -latomic -lsmime3 -lnss3 -lnssutil3 -lplds4 -lplc4 -lnspr4 -lpng16 -lpangocairo-1.0 -lpango-1.0 -lcairo -lfontconfig -lexpat -lz -ljpeg -lwebp -lwebpdemux -lgio-2.0
/usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: warning: libfreetype.so.6, needed by /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../lib64/libfontconfig.so, may conflict with libfreetype.so.6
obj/skia/libskia.a(SkWebpEncoder.o): In function `do_encode(SkWStream*, SkPixmap const&, SkWebpEncoder::Options const&)':
SkWebpEncoder.cpp:(.text._ZL9do_encodeP9SkWStreamRK8SkPixmapRKN13SkWebpEncoder7OptionsE+0x46c): undefined reference to `WebPNewInternal'
SkWebpEncoder.cpp:(.text._ZL9do_encodeP9SkWStreamRK8SkPixmapRKN13SkWebpEncoder7OptionsE+0x47e): undefined reference to `WebPMuxSetImage'
SkWebpEncoder.cpp:(.text._ZL9do_encodeP9SkWStreamRK8SkPixmapRKN13SkWebpEncoder7OptionsE+0x494): undefined reference to `WebPMuxDelete'
SkWebpEncoder.cpp:(.text._ZL9do_encodeP9SkWStreamRK8SkPixmapRKN13SkWebpEncoder7OptionsE+0x68e): undefined reference to `WebPMuxSetChunk'
SkWebpEncoder.cpp:(.text._ZL9do_encodeP9SkWStreamRK8SkPixmapRKN13SkWebpEncoder7OptionsE+0x6aa): undefined reference to `WebPMuxAssemble'
SkWebpEncoder.cpp:(.text._ZL9do_encodeP9SkWStreamRK8SkPixmapRKN13SkWebpEncoder7OptionsE+0x6f4): undefined reference to `WebPMuxDelete'
collect2: error: ld returned 1 exit status

This does not affect any build at Google. build/linux/unbundle
is a completely parallel directory tree intended to be used
by Linux distribution packagers. Please see
https://cs.chromium.org/chromium/src/build/linux/unbundle/README
for more context.

BUG=none
R=thestig@chromium.org

Review-Url: https://codereview.chromium.org/2885113003 .
Cr-Original-Commit-Position: refs/heads/master@{#472532}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a9c4826790de16b317538db629b44069fa77ddc4

linux/unbundle/libwebp.gn

@@ -6,5 +6,6 @@ source_set("libwebp") {
   libs = [
     "webp",
     "webpdemux",
+    "webpmux",
   ]
 }