DevTools: add Tracing agent on back-end

Source/devtools/protocol.json

 {
     "version": { "major": "1", "minor": "1" },
     "domains": [{
         "domain": "Inspector",
         "hidden": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications."
@@ skipping 2755 matching lines @@
                     { "name": "thread", "type": "string", "optional": true, "hidden": true, "description": "If present, identifies the thread that produced the event." },
                     { "name": "stackTrace", "$ref": "Console.StackTrace", "optional": true, "hidden": true, "description": "Stack trace." },
                     { "name": "frameId", "type": "string", "optional": true, "hidden": true, "description": "Unique identifier of the frame within the page that the event relates to." }
                 ],
                 "description": "Timeline record contains information about the recorded activity."
             }
         ],
         "commands": [
             {
                 "name": "enable",
-                "description": "Enables timeline. After this call, timeline can be started from within the page (for example upon console.timeline).",
-                "parameters": [
-                    { "name": "traceEventCategoryFilter", "optional": true, "type": "string", "description": "If set, the timeline agent will automatically start tracing with the specified category filter when timeline is started." }
-                ]
+                "description": "Enables timeline. After this call, timeline can be started from within the page (for example upon console.timeline)."
             },
             {
                 "name": "disable",
                 "description": "Disables timeline."
             },
             {
                 "name": "start",
                 "parameters": [
                     { "name": "maxCallStackDepth", "optional": true, "type": "integer", "description": "Samples JavaScript stack traces up to <code>maxCallStackDepth</code>, defaults to 5." },
                     { "name": "bufferEvents", "optional": true, "type": "boolean", "hidden": true, "description": "Whether instrumentation events should be buffered and returned upon <code>stop</code> call." },
@@ skipping 1235 matching lines @@
             }
         ]
     },
     {
         "domain": "Tracing",
         "hidden": true,
         "commands": [
             {
                 "name": "start",
                 "description": "Start trace events collection.",
-                "async": true,
                 "parameters": [
                     { "name": "categories", "type": "string", "description": "Category/tag filter" },
                     { "name": "options", "type": "string", "description": "Tracing options" },
                     { "name": "bufferUsageReportingInterval", "type": "number", "optional": true, "description": "If set, the agent will issue bufferUsage events at this interval, specified in milliseconds" }
                 ],
-                "handlers": ["browser", "frontend"]
+                "returns": [
+                    { "name": "sessionId", "type": "string", "description": "A system-unique identifier of the tracing session that allows associating of some of the trace events with the inspected page" }
+                ],
+                "handlers": ["browser", "renderer", "frontend"]
             },
             {
                 "name": "end",
                 "description": "Stop trace events collection.",
                 "handlers": ["browser", "frontend"]
             }
         ],
         "events": [
             {
                 "name": "dataCollected",
                 "parameters": [
                     { "name": "value", "type": "array", "items": { "type": "object" } }
                 ],
+                "description": "Contains an bucket of collected trace events.",
                 "handlers": ["browser", "frontend"]
             },
             {
                 "name": "tracingComplete",
+                "description": "Signals that tracing is stopped and there is no trace buffers pending flush, all data were delivered via dataCollected events.",
                 "handlers": ["browser", "frontend"]
             },
             {
                 "name": "bufferUsage",
                 "parameters": [
                     { "name": "value", "type": "number", "description": "A number in range [0..1] that indicates the used size of event buffer as a fraction of its total size." }
                 ],
                 "handlers": ["browser", "frontend"]
             }
         ]
@@ skipping 40 matching lines @@
             {
                 "name": "dataAvailable",
                 "parameters": [
                     {"name": "value", "type": "array", "items": { "$ref": "PowerEvent" }, "description": "List of power events." }
                 ],
                 "handlers": ["browser", "frontend"]
             }
         ]
     }]
 }