Fix compile on mac: format string mismatch error.

See http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_compile_dbg/builds/9542/steps/compile%20%28with%20patch%29/logs/stdio

TBR=jun_fang@foxitsoftware.com,

Review URL: https://codereview.chromium.org/506053003