Android perf: force flushing stdout.

Some of the perf bots are still not showing the output.
Force flushing the stdout,

BUG=320445
NOTRY=True

Review URL: https://codereview.chromium.org/67463004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235995 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235995