[DevTools] Make all parameters of Tracing.start optional.

Source/devtools/protocol.json

 {
     "version": { "major": "1", "minor": "1" },
     "domains": [{
         "domain": "Inspector",
         "hidden": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications."
@@ skipping 4293 matching lines @@
     },
     {
         "domain": "Tracing",
         "hidden": true,
         "commands": [
             {
                 "name": "start",
                 "async": true,
                 "description": "Start trace events collection.",
                 "parameters": [
-                    { "name": "categories", "type": "string", "description": "Category/tag filter" },
-                    { "name": "options", "type": "string", "description": "Tracing options" },
+                    { "name": "categories", "type": "string", "optional": true, "description": "Category/tag filter" },
+                    { "name": "options", "type": "string", "optional": true, "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", "renderer"]
             },
             {
                 "name": "end",
                 "async": true,
                 "description": "Stop trace events collection.",
                 "handlers": ["browser", "renderer"]
             },
@@ skipping 179 matching lines @@
                     { "name": "isRunning", "type": "boolean", "description": "If the animation player is still running." }
                 ],
                 "description": "Gets the state of an AnimationPlayer.",
                 "hidden": true
             }
         ],
         "events": [
         ]
     }]
 }