Default to building instead of just linking libpng for Android

Right now, the default defined in libpng.gyp is to use the system's
libpng for Android, while we want to build this from source.

BUG=
TEST=

Review URL: http://codereview.chromium.org/8486008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@110308 0039d316-1c4b-4281-b951-d872f2087c98

third_party/libpng/libpng.gyp

@@ -5,7 +5,7 @@
 {
   'variables': {
     'conditions': [
-      [ 'os_posix == 1 and OS != "mac"', {
+      [ 'os_posix == 1 and OS != "mac" and OS != "android"', {
         # Link to system .so since we already use it due to GTK.
         'use_system_libpng%': 1,
       }, {  # os_posix != 1 or OS == "mac"