Fix for Android GN build.

Fix for Android GN build.

is_linux condition excludes is_android and causes the following error:

In file included from
../../third_party/hunspell/src/hunspell/affentry.cxx:9:0:
../../third_party/hunspell/src/hunspell/affentry.hxx:30:105: error:
converting to non-pointer type 'short unsigned int' from NULL
[-Werror=conversion-null]
   struct hentry *      check_twosfx(const char * word, int len, char
                   in_compound, const FLAG needflag = NULL);

http://build.chromium.org/p/tryserver.chromium/builders/android_chromium_gn_compile_rel/builds/16090

So we switch back to is_posix condition, which should cover Android as
well.

To test an Android build:

$ gn gen out/Debug_gn --args='os="android" cpu_arch="arm"'
$ ninja -C out/Debug_gn

BUG=None
TEST=None
R=brettw@chromium.org

Committed: 278973

[brettw, 2014-06-21 05:36:34]

lgtm

[tfarina, 2014-06-21 13:53:50]

Committed patchset #1 manually as r278973 (presubmit successful).