Android: configure use_system_zlib in zlib.gyp

Currently third_party/zlib/zlib.gyp uses the system libz if:
  os_posix == 1 and OS != "mac" and OS != "ios" and OS != "openbsd"

I'm assuming that Android sets os_posix==1 and therefore this change is
a no-op. But it consolidates control of this variable in a single
location.

BUG=none

https://chromiumcodereview.appspot.com/10837100/

git-svn-id: http://src.chromium.org/svn/trunk/src/build@150355 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -987,9 +987,6 @@
         'use_system_skia%': '0',
         'use_system_harfbuzz%': '0',
 
-        # Always use the system zlib.
-        'use_system_zlib%': 1,
-
         # Configure crash reporting and build options based on release type.
         'conditions': [
           ['buildtype=="Official"', {