Fixed compile_frontend.py warnings.

Source/devtools/protocol.json

 {
     "version": { "major": "1", "minor": "0" },
     "domains": [{
         "domain": "Inspector",
         "hidden": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications."
@@ skipping 3036 matching lines @@
                     { "name": "location", "$ref": "Location", "description": "Actual breakpoint location." }
                 ],
                 "description": "Fired when breakpoint is resolved to an actual script and location."
             },
             {
                 "name": "paused",
                 "parameters": [
                     { "name": "callFrames", "type": "array", "items": { "$ref": "CallFrame" }, "description": "Call stack the virtual machine stopped on." },
                     { "name": "reason", "type": "string", "enum": [ "XHR", "DOM", "EventListener", "exception", "assert", "CSPViolation", "other" ], "description": "Pause reason." },
                     { "name": "data", "type": "object", "optional": true, "description": "Object containing break-specific auxiliary properties." },
-                    { "name": "hitBreakpoints", "type": "array", "items": { "type": "string" }, "description": "Hit breakpoints IDs", "hidden": true }
+                    { "name": "hitBreakpoints", "type": "array", "optional": true, "items": { "type": "string" }, "description": "Hit breakpoints IDs", "hidden": true }
                 ],
                 "description": "Fired when the virtual machine stopped on breakpoint or exception or any other stop criteria."
             },
             {
                 "name": "resumed",
                 "description": "Fired when the virtual machine resumed execution."
             }
         ]
     },
     {
@@ skipping 688 matching lines @@
                 ]
             }
         ],
         "events": [
             {
                 "name": "layerTreeDidChange"
             }
         ]
     }]
 }