Only use devtools category for devtools instrumentation trace events

Only use devtools category for devtools instrumentation trace events

- change PlatformInstrumentation events to use "devtools" as category
- change category filter to only include "devtools"

BUG=
R=pfeldman@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=169445

[pfeldman, 2014-03-13 05:25:01]

lgtm

[caseq, 2014-03-13 05:30:30]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2014-03-13 05:30:36]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/caseq@chromium.org/180783016/1

[commit-bot: I haz the power, 2014-03-13 06:12:51]

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

[commit-bot: I haz the power, 2014-03-13 06:12:52]

Try jobs failed on following builders:
  tryserver.chromium on win_blink_rel

[caseq, 2014-03-18 13:54:55]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2014-03-18 13:54:57]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/caseq@chromium.org/180783016/1

[caseq, 2014-03-18 14:04:11]

Committed patchset #1 manually as r169445 (presubmit successful).