Move blink_perf.xml_http_request benchmark to tools/perf/contrib/ directory

* I verified that people still be able to run "./run_benchmark --browser=system blink_perf.xml_http_request" locally.
* Try bot run are in: https://build.chromium.org/p/tryserver.chromium.perf/builders/mac_10_12_perf_bisect/builds/149

Other effects of this:
_ People can run blink_perf.xml_http_request locally without needing to
pass "--also-run-disabled-benchmark"
_ The benchmarks' no-op run is no longer scheduled in perf waterfall,
hence the size reduction to the testing/buildbot/chrome.perf.* files

BUG=709063

Review-Url: https://codereview.chromium.org/2840213003
Cr-Original-Commit-Position: refs/heads/master@{#467347}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: bfe47df90b3d1d770f9a2dcef1bd83b966327ce4
2 files changed