Build fix for Android

Sometimes _POSIX_THREAD_KEYS_MAX will already be defined

TBR=cevans@chromium.org
BUG=

Review URL: https://codereview.chromium.org/63653002

git-svn-id: svn://svn.chromium.org/blink/trunk@161500 bbb929c8-8fbe-4397-9dbb-9b2b20218538