set processor affinity on adb during run-webkit-tests on Android.

R=tonyg@chromium.org, peter@chromium.org
BUG=305362

Review URL: https://codereview.chromium.org/27643002

git-svn-id: svn://svn.chromium.org/blink/trunk@159911 bbb929c8-8fbe-4397-9dbb-9b2b20218538