DevTools: show progress when retrieving recorded trace events

Source/devtools/protocol.json

 {
     "version": { "major": "1", "minor": "1" },
     "domains": [{
         "domain": "Inspector",
         "hidden": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications."
@@ skipping 4360 matching lines @@
                 "handlers": ["browser"]
             },
             {
                 "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"]
             },
             {
                 "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." }
+                    { "name": "percentFull", "type": "number", "optional": true, "description": "A number in range [0..1] that indicates the used size of event buffer as a fraction of its total size." },
+                    { "name": "eventCount", "type": "number", "optional": true, "description": "An approximate number of events in the trace log." },
+                    { "name": "value", "type": "number", "optional": true, "description": "A number in range [0..1] that indicates the used size of event buffer as a fraction of its total size." }
                 ],
                 "handlers": ["browser"]
             }
         ]
     },
     {
         "domain": "Power",
         "hidden": true,
         "types": [
             {
@@ skipping 162 matching lines @@
                     { "name": "isRunning", "type": "boolean", "description": "If the animation player is still running." }
                 ],
                 "description": "Gets the state of an AnimationPlayer.",
                 "hidden": true
             }
         ],
         "events": [
         ]
     }]
 }