Fix libwebp unbundle

BUG=805488
R=thestig

Change-Id: I577bf4f5682cffced4f7424768545cc467ad6416
Reviewed-on: https://chromium-review.googlesource.com/884942Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531753}

build/linux/unbundle/libwebp.gn

@@ -2,10 +2,16 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-source_set("libwebp") {
+source_set("libwebp_webp") {
   libs = [
     "webp",
     "webpdemux",
     "webpmux",
   ]
 }
+
+group("libwebp") {
+  deps = [
+    ":libwebp_webp",
+  ]
+}