Add new request type TIMING for clock sync requests

[pcruiksh]

Adds shaka.net.NetworkingEngine.RequestType.TIMING for identification of clock sync requests in networking filters. Fixes #1488.

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[googlebot]

CLAs look good, thanks!

[joeyparrish]

Looks good, but can you please update CONTRIBUTORS and AUTHORS as well? There are notes at the top about how to do that, but please make sure to keep them alphabetized. Thanks!

[joeyparrish]

Looks good to me. Thanks!

[joeyparrish]

I'm just going to have the build bot check it out. It's not doing anything magical, so it if it reports any failures, just run build/all.py and build/test.py locally to reproduce. If all the tests pass, I'll merge it.

[shaka-bot]

Test Failure:

Chrome 67.0.3396 (Linux 0.0.0)
  DashParser Live stop interrupts UTCTiming requests FAILED
    Expected spy request to have been called with [ 0, <jasmine.objectContaining(Object({ uris: [ 'http://foo.bar/date' ] }))> ] but actual calls were [ 4, Object({ uris: [ 'http://foo.bar/date' ], method: 'GET', body: null, headers: Object({  }), allowCrossSiteCredentials: false, retryParameters: Object({ maxAttempts: 2, baseDelay: 1000, backoffFactor: 2, fuzzFactor: 0.5, timeout: 0 }) }) ].
        at Function.shaka.test.FakeNetworkingEngine.expectRequest (test/test/util/fake_networking_engine.js:91:22)
        at shaka.test.FakeNetworkingEngine.expectRequest (test/test/util/fake_networking_engine.js:215:35)
        at test/dash/dash_parser_live_unit.js:894:23
        at eval (node_modules/promise-polyfill/lib/index.js:61:13)

Failed 1 tests, passed 1352, skipped 51

[joeyparrish]

Looks like one test needs updating. You can check results locally with build/all.py (to lint and compile everything) and build/test.py (to run the tests). Thanks!

[shaka-bot]

All tests passed!

[joeyparrish]

Thanks for your contribution!