DevTools: handle Tracing.start in both browser and renderer

DevTools: handle Tracing.start in both browser and renderer

DevToolsTracingHandler is used for inspecting two different targets: browser and renderer. The former one is used by Telemetry and behavior in that case hasn't changed.

When the handler is used for inslecting renderer we do not return response from browser override for Tracing.start so that InspectorTracingAgent in the render process could handle it.
For blink counterpart, see: https://codereview.chromium.org/254613002

BUG=361045, 374411
R=caseq@chromium.org, pfeldman@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=271368

[pfeldman, 2014-05-19 06:19:47]

lgtm

[caseq, 2014-05-19 06:28:55]

lgtm

[yurys, 2014-05-19 06:35:01]

The CQ bit was checked by yurys@chromium.org

[commit-bot: I haz the power, 2014-05-19 06:35:17]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/yurys@chromium.org/295613002/1

[commit-bot: I haz the power, 2014-05-19 08:34:23]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  android_dbg_triggered_tests on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_dbg_triggered...)

[commit-bot: I haz the power, 2014-05-19 09:28:05]

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

[commit-bot: I haz the power, 2014-05-19 09:28:06]

Try jobs failed on following builders:
  android_dbg_triggered_tests on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_dbg_triggered...)

[yurys, 2014-05-19 12:47:32]

Committed patchset #1 manually as r271368 (presubmit successful).