Timeline: produce session cookie when timeline is started

Source/devtools/protocol.json

 {
     "version": { "major": "1", "minor": "1" },
     "domains": [{
         "domain": "Inspector",
         "hidden": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications."
@@ skipping 2790 matching lines @@
             {
                 "name": "progress",
                 "parameters": [
                     { "name": "count", "type": "number" }
                 ],
                 "hidden": true
             },
             {
                 "name": "started",
                 "parameters": [
+                    { "name": "sessionId", "type": "string", "description": "A system-unique identifier that is emitted in some timeline trace events to distinguish events coming from the inspected page." },
                     { "name": "consoleTimeline", "type": "boolean", "optional": true, "description": "If specified, identifies that timeline was started using console.timeline() call." }
                 ],
                 "description": "Fired when timeline is started.",
                 "hidden": true
             },
             {
                 "name": "stopped",
                 "parameters": [
                     { "name": "consoleTimeline", "type": "boolean", "optional": true, "description": "If specified, identifies that timeline was started using console.timeline() call." }
                 ],
@@ skipping 1277 matching lines @@
             {
                 "name": "dataAvailable",
                 "parameters": [
                     {"name": "value", "type": "array", "items": { "$ref": "PowerEvent" }, "description": "List of power events." }
                 ],
                 "handlers": ["browser", "frontend"]
             }
         ]
     }]
 }