Remove start & stop logic from timeline_metric.

Move start & stop logic from timeline_metric to measurement/. This is the first step in moving
timeline and thread_time to timeline_based_metric API.

BUG=345922
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=259710

[nduca, 2014-03-26 17:36:21]

lgtm

[nednguyen, 2014-03-26 17:37:11]

The CQ bit was checked by nednguyen@google.com

[commit-bot: I haz the power, 2014-03-26 17:39:00]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/nednguyen@google.com/199733007/1

[commit-bot: I haz the power, 2014-03-26 18:32:08]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-26 18:32:09]

Retried try job too often on win_rel for step(s) app_list_unittests,
ash_unittests, aura_unittests, cacheinvalidation_unittests, cc_unittests,
check_deps, check_deps2git, chrome_elf_unittests, chromedriver_unittests,
components_unittests, compositor_unittests, content_browsertests,
content_unittests, crypto_unittests, events_unittests, gfx_unittests,
google_apis_unittests, gpu_unittests, installer_util_unittests, ipc_tests,
jingle_unittests, media_unittests, mini_installer_test, nacl_integration,
ppapi_unittests, printing_unittests, remoting_unittests, sql_unittests,
sync_integration_tests, sync_unit_tests, telemetry_perf_unittests,
telemetry_unittests, views_unittests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[nednguyen, 2014-03-26 18:48:42]

The CQ bit was checked by nednguyen@google.com

[commit-bot: I haz the power, 2014-03-26 18:51:44]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/nednguyen@google.com/199733007/1

[commit-bot: I haz the power, 2014-03-26 22:05:08]

Change committed as 259710