[Telemetry] Attempt to fix flaky jsgamebench timeouts.

This benchmark can take 30 mins to run, but the buildbot times out after 20 mins
of no output. So raise its timeout and change WaitFor to output a heartbeat so
that the buildbot is happy with longer timeouts.

BUG=247476

Review URL: https://codereview.chromium.org/95513003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237833 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237833