[Telemetry] Make chrome_tracing_agent state more resilient to tracing exception.

[Telemetry] Make chrome_tracing_agent state more resilient to tracing exception.

Because ChromeTracingAgent class has global states, when any
exception was raised inside StopTracing method, the active tracing
state of this class is not set to False, making subsequent unittest that
try to StartTracing fails because of this.

Long term fix of this depends on crbug.com/317481 be fixed so that
ChromeTracingAgent no longer have global state.

BUG=459807
Committed: https://crrev.com/0f07fbef765278a5da7e21741eb2775419035a4a
Cr-Commit-Position: refs/heads/master@{#320799}

[nednguyen, 2015-03-13 02:45:51]

Patchset #2 (id:20001) has been deleted

[nednguyen, 2015-03-13 02:52:43]

Patchset #1 (id:1) has been deleted

[nednguyen, 2015-03-13 02:52:48]

Patchset #1 (id:1) has been deleted

[nednguyen, 2015-03-13 02:53:05]

Patchset #1 (id:1) has been deleted

[nednguyen, 2015-03-13 17:19:32]

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

[nednguyen, 2015-03-13 20:22:57]

Patchset #2 (id:60001) has been deleted

[nednguyen, 2015-03-13 20:23:05]

Patchset #1 (id:40001) has been deleted

[Zhen Wang, 2015-03-13 21:53:30]

lg2m

[nednguyen, 2015-03-14 01:14:05]

The CQ bit was checked by nednguyen@google.com

[commit-bot: I haz the power, 2015-03-14 01:14:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/997193002/80001

[commit-bot: I haz the power, 2015-03-14 01:14:42]

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

[commit-bot: I haz the power, 2015-03-14 01:14:43]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[nednguyen, 2015-03-16 17:42:51]

On 2015/03/14 01:14:43, I haz the power (commit-bot) wrote:
> No LGTM from a valid reviewer yet. Only full committers are accepted.
> Even if an LGTM may have been provided, it was from a non-committer,
> _not_ a full super star committer.
> See http://www.chromium.org/getting-involved/become-a-committer
> Note that this has nothing to do with OWNERS files.

PTAL.

[nednguyen, 2015-03-16 19:39:23]

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

[sullivan, 2015-03-16 20:31:06]

lgtm

[nednguyen, 2015-03-16 20:31:20]

The CQ bit was checked by nednguyen@google.com

[commit-bot: I haz the power, 2015-03-16 20:31:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/997193002/80001

[commit-bot: I haz the power, 2015-03-16 21:59:41]

Committed patchset #1 (id:80001)

[commit-bot: I haz the power, 2015-03-16 22:00:16]

Patchset 1 (id:??) landed as
https://crrev.com/0f07fbef765278a5da7e21741eb2775419035a4a
Cr-Commit-Position: refs/heads/master@{#320799}