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
Review URL: https://codereview.chromium.org/
295613002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271368 0039d316-1c4b-4281-b951-d872f2087c98