Fix Chromium tracing to not override app logging

Fix Chromium tracing to not override app logging

Modify TraceEvent to only override tracing when it has been previously set by
Chromium, not for when other apps have set it.

BUG=651215
Committed: https://crrev.com/0c54bb9b87180c4eb1fea08afae89848111ea3ad
Cr-Commit-Position: refs/heads/master@{#422016}

[Nate Fischer, 2016-09-28 21:25:52]

Description was changed from

==========
Fix Chromium tracing to not override app logging

Modify TraceEvent to only override tracing when it has been previously set by
Chromium, not for when other apps have set it.

BUG=
==========

to

==========
Fix Chromium tracing to not override app logging

Modify TraceEvent to only override tracing when it has been previously set by
Chromium, not for when other apps have set it.

BUG=651215
==========

[Nate Fischer, 2016-09-28 21:27:57]

ntfschr@chromium.org changed reviewers:
+ torne@chromium.org

[Nate Fischer, 2016-09-28 22:14:12]

I've verified this change on an Android N device by following these steps (taken
from the internal bug report):

1. Tap the search plate, type a couple of characters
2. Take an android GSA system dump (adb shell dumpsys activity service
SearchService)
3. Type some more characters
4. Take another system dump

Before making this change, the two system dumps would be identical in the "All
UI Tasks" section. After this change, the "All UI Tasks" sections differ, with
the later system dump having more content at the end.

Let me know if this is the kind of thing we want a unit test for, or if we
should test this more thoroughly.

I originally had this as a new static variable, but decided that sEnabled
probably covers all the cases that I would need.

[Torne, 2016-09-29 15:42:44]

Can you add a comment in the code that briefly explains that we need to only
change it when the value actually changes to avoid disrupting application usage
in webview? With that, this LGTM.

I think it's sufficiently difficult to add an automated test that verifies apps
aren't impacted that it's not worth doing; this code is pretty simple and as
long as we make it clear why it works this way it's unlikely to get broken.

[Nate Fischer, 2016-09-30 01:13:54]

The CQ bit was checked by ntfschr@chromium.org

[Nate Fischer, 2016-09-30 01:13:55]

The patchset sent to the CQ was uploaded after l-g-t-m from torne@chromium.org
Link to the patchset: https://codereview.chromium.org/2380563003/#ps80001
(title: "Improving comment")

[Nate Fischer, 2016-09-30 01:14:08]

On 2016/09/29 at 15:42:44, torne wrote:
> Can you add a comment in the code that briefly explains that we need to only
change it when the value actually changes to avoid disrupting application usage
in webview? With that, this LGTM.
> 
> I think it's sufficiently difficult to add an automated test that verifies
apps aren't impacted that it's not worth doing; this code is pretty simple and
as long as we make it clear why it works this way it's unlikely to get broken.

Done.

[commit-bot: I haz the power, 2016-09-30 01:14:27]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-09-30 02:46:51]

Committed patchset #5 (id:80001)

[commit-bot: I haz the power, 2016-09-30 02:48:29]

Description was changed from

==========
Fix Chromium tracing to not override app logging

Modify TraceEvent to only override tracing when it has been previously set by
Chromium, not for when other apps have set it.

BUG=651215
==========

to

==========
Fix Chromium tracing to not override app logging

Modify TraceEvent to only override tracing when it has been previously set by
Chromium, not for when other apps have set it.

BUG=651215

Committed: https://crrev.com/0c54bb9b87180c4eb1fea08afae89848111ea3ad
Cr-Commit-Position: refs/heads/master@{#422016}
==========

[commit-bot: I haz the power, 2016-09-30 02:48:30]

Patchset 5 (id:??) landed as
https://crrev.com/0c54bb9b87180c4eb1fea08afae89848111ea3ad
Cr-Commit-Position: refs/heads/master@{#422016}