Deflake http/tests/navigation/ping-*.

This CL de-flakes the http/tests/navigation/ping-* suite by ensuring
that each test generates its own ping file dump, and that the temp
directory is consistently used.

Review URL: https://codereview.chromium.org/334113003

git-svn-id: svn://svn.chromium.org/blink/trunk@176335 bbb929c8-8fbe-4397-9dbb-9b2b20218538