[Telemetry] Clear tracing state before browser start

[Telemetry] Clear tracing state before browser start

For TimelineBasedMeasurement benchmarks, tracing has already 
started before starting browser. For PageTest benchmarks, tracing has 
not yet started before starting browser. We need to make sure no 
tracing state is left before starting the browser for PageTest 
benchmarks.

BUG=chromium:596464
Committed: https://chromium.googlesource.com/external/github.com/catapult-project/catapult/+/bc0655db4a8ec68d1b180907e235eeaf17dbb33a

[Zhen Wang, 2016-03-25 04:59:34]

Description was changed from

==========
[Telemetry] Clear tracing state before browser start

BUG=chromium:#596464
==========

to

==========
[Telemetry] Clear tracing state before browser start

BUG=chromium:596464
==========

[Zhen Wang, 2016-03-25 05:01:12]

Description was changed from

==========
[Telemetry] Clear tracing state before browser start

BUG=chromium:596464
==========

to

==========
[Telemetry] Clear tracing state before browser start

For TimelineBasedMeasurement benchmarks, tracing has already 
started before starting browser. For PageTest benchmarks, tracing has 
not yet started before starting browser. We need to make sure no 
tracing state is left before starting the browser for PageTest 
benchmarks.

BUG=chromium:596464
==========

[Zhen Wang, 2016-03-25 15:04:47]

zhenw@chromium.org changed reviewers:
+ nednguyen@google.com

[Zhen Wang, 2016-03-25 15:04:47]

ptal

[nednguyen, 2016-03-25 15:19:31]

The CQ bit was checked by nednguyen@google.com

[nednguyen, 2016-03-25 15:19:31]

lgtm

[commit-bot: I haz the power, 2016-03-25 15:19:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1827323003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1827323003/20001

[commit-bot: I haz the power, 2016-03-25 15:32:33]

Description was changed from

==========
[Telemetry] Clear tracing state before browser start

For TimelineBasedMeasurement benchmarks, tracing has already 
started before starting browser. For PageTest benchmarks, tracing has 
not yet started before starting browser. We need to make sure no 
tracing state is left before starting the browser for PageTest 
benchmarks.

BUG=chromium:596464
==========

to

==========
[Telemetry] Clear tracing state before browser start

For TimelineBasedMeasurement benchmarks, tracing has already 
started before starting browser. For PageTest benchmarks, tracing has 
not yet started before starting browser. We need to make sure no 
tracing state is left before starting the browser for PageTest 
benchmarks.

BUG=chromium:596464

Committed:
https://chromium.googlesource.com/external/github.com/catapult-project/catapu...
==========

[commit-bot: I haz the power, 2016-03-25 15:32:34]

Committed patchset #2 (id:20001) as
https://chromium.googlesource.com/external/github.com/catapult-project/catapu...

[nednguyen, 2016-04-11 18:12:29]

https://codereview.chromium.org/1827323003/diff/20001/telemetry/telemetry/pag...
File telemetry/telemetry/page/shared_page_state.py (right):

https://codereview.chromium.org/1827323003/diff/20001/telemetry/telemetry/pag...
telemetry/telemetry/page/shared_page_state.py:224: if not
self.platform.tracing_controller.is_tracing_running:
Actually, I think we should move this to browser.Start() method.