Add Tracing.getCategories to protocol.json.

Source/devtools/protocol.json

 {
     "version": { "major": "1", "minor": "1" },
     "domains": [{
         "domain": "Inspector",
         "hidden": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications."
@@ skipping 4108 matching lines @@
                 ],
                 "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"]
+            },
+            {
+                "name": "getCategories",
+                "description": "Gets supported tracing categories.",
+                "returns": [
+                    { "name": "categories", "type": "array", "items": { "type": "string" }, "description": "A list of supported tracing categories." }
+                ],
+                "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"]
@@ skipping 54 matching lines @@
             {
                 "name": "dataAvailable",
                 "parameters": [
                     {"name": "value", "type": "array", "items": { "$ref": "PowerEvent" }, "description": "List of power events." }
                 ],
                 "handlers": ["browser", "frontend"]
             }
         ]
     }]
 }