GN: Change libpng to be a group rather than a static_library, since it has no sources.

third_party/libpng/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 config("libpng_config") {
   include_dirs = [
     ".",
   ]
 
   defines = [
@@ skipping 52 matching lines @@
       defines = [ "PNG_BUILD_DLL" ]
     }
   }
 
   public_configs = [ ":libpng_config" ]
 
   public_deps = [ "//third_party/zlib" ]
 }
 
 if (is_win) {
   component("libpng") {

[Robert Sesek, 2014/09/24 22:28:54]

Is it correct to use public_deps with component() like this? If not, I'll change
all OSes to use group().

[brettw, 2014/09/24 22:30:56]

Yes, this is fine. An argument could be made that all OS's should just be a
source_set, but this will end up duplicating libpng some number of times and
this matches the GYP build, so I'm inclined to keep it as-is.

     public_deps = [ ":libpng_sources" ]
   }
 } else {
-  static_library("libpng") {
+  group("libpng") {
     public_deps = [ ":libpng_sources" ]
   }
 }