[DevTools] Make unused ScreencastFrameMetadata properties 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 191 matching lines @@
                     { "name": "minimumPageScaleFactor", "type": "number", "description": "Minimum page scale factor." },
                     { "name": "maximumPageScaleFactor", "type": "number", "description": "Maximum page scale factor." }
                 ],
                 "hidden": true
             },
             {
                 "id": "ScreencastFrameMetadata",
                 "type": "object",
                 "description": "Screencast frame metadata",
                 "properties": [
-                    { "name": "deviceScaleFactor", "type": "number", "hidden": true, "description": "Device scale factor." },
-                    { "name": "viewport", "$ref": "DOM.Rect", "hidden": true, "description": "Viewport in CSS pixels." },
-                    { "name": "offsetTop", "type": "number", "hidden": true, "optional": true, "description": "Top offset in DIP." },
+                    { "name": "deviceScaleFactor", "type": "number", "hidden": true, "optional": true, "description": "Device scale factor." },
+                    { "name": "viewport", "$ref": "DOM.Rect", "hidden": true, "optional": true, "description": "Viewport in CSS pixels." },
+                    { "name": "offsetTop", "type": "number", "hidden": true, "description": "Top offset in DIP." },
                     { "name": "offsetBottom", "type": "number", "hidden": true, "optional": true, "description": "Bottom offset in DIP." },
                     { "name": "pageScaleFactor", "type": "number", "hidden": true, "description": "Page scale factor." },
-                    { "name": "pageScaleFactorMin", "type": "number", "hidden": true, "description": "Page scale factor min." },
-                    { "name": "pageScaleFactorMax", "type": "number", "hidden": true, "description": "Page scale factor max." },
+                    { "name": "pageScaleFactorMin", "type": "number", "hidden": true, "optional": true, "description": "Page scale factor min." },
+                    { "name": "pageScaleFactorMax", "type": "number", "hidden": true, "optional": true, "description": "Page scale factor max." },
                     { "name": "deviceWidth", "type": "number", "hidden": true, "description": "Device screen width in DIP." },
                     { "name": "deviceHeight", "type": "number", "hidden": true, "description": "Device screen height in DIP." },
                     { "name": "scrollOffsetX", "type": "number", "hidden": true, "description": "Position of horizontal scroll in CSS pixels." },
                     { "name": "scrollOffsetY", "type": "number", "hidden": true, "description": "Position of vertical scroll in CSS pixels." }
                 ],
                 "hidden": true
             }
         ],
         "commands": [
             {
@@ skipping 4305 matching lines @@
                     { "name": "isRunning", "type": "boolean", "description": "If the animation player is still running." }
                 ],
                 "description": "Gets the state of an AnimationPlayer.",
                 "hidden": true
             }
         ],
         "events": [
         ]
     }]
 }