[Blink] [PowerProfiler] Support accuracy level attribute for power data provider.

Source/devtools/protocol.json

 {
     "version": { "major": "1", "minor": "1" },
     "domains": [{
         "domain": "Inspector",
         "hidden": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications."
@@ skipping 4175 matching lines @@
                 "handlers": ["browser", "frontend"]
             },
             {
                 "name": "canProfilePower",
                 "description": "Tells whether power profiling is supported.",
                 "returns": [
                   { "name": "result", "type": "boolean", "description": "True if power profiling is supported." }
                 ],
                 "hidden": true,
                 "handlers": ["browser", "frontend"]
+            },
+            {
+                "name": "getAccuracyLevel",
+                "description": "Describes the accuracy level of the data provider.",
+                "returns": [
+                    { "name": "result", "type": "string", "enum": ["high", "moderate", "low"] }
+                ],
+                "handlers": ["browser", "frontend"]
             }
         ],
         "events": [
             {
                 "name": "dataAvailable",
                 "parameters": [
                     {"name": "value", "type": "array", "items": { "$ref": "PowerEvent" }, "description": "List of power events." }
                 ],
                 "handlers": ["browser", "frontend"]
             }
         ]
     }]
 }