DevTools: handle Tracing.start in both browser and renderer

DevTools: handle Tracing.start in both browser and renderer

Do not return response from browser override for Tracing.start, so
we invoke one in the renderer.
For blink counterpart, see: https://codereview.chromium.org/254613002

BUG=361045
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=267126

[caseq, 2014-04-24 11:54:39]

We now expect callback for Tracing.start() to be invoked from the back-end, when
the tracing is enabled there.

[yurys, 2014-04-24 14:15:49]

lgtm

[caseq, 2014-04-29 11:27:51]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2014-04-29 11:28:02]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/caseq@chromium.org/256473004/1

[commit-bot: I haz the power, 2014-04-29 11:53:48]

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

[commit-bot: I haz the power, 2014-04-29 11:53:49]

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

[caseq, 2014-04-29 16:02:39]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2014-04-29 16:03:28]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/caseq@chromium.org/256473004/1

[commit-bot: I haz the power, 2014-04-29 22:06:57]

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

[commit-bot: I haz the power, 2014-04-29 22:06:57]

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

[caseq, 2014-04-30 07:55:03]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2014-04-30 07:55:29]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/caseq@chromium.org/256473004/1

[commit-bot: I haz the power, 2014-04-30 07:58:59]

Change committed as 267126