[GN] Build fix on unused return value warning.

Older version of stdio.h in glibc (before 2.17) has warn_unused_result attribute on fwrite.
That causes build failure.
This CL change suppress the warning.

NOTRY=true

Review URL: https://codereview.chromium.org/26587006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227721 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227721