[DevTools] Definitions of capabilities in protocol.

Source/devtools/protocol.json

 {
     "version": { "major": "1", "minor": "1" },
     "domains": [{
         "domain": "Inspector",
         "hidden": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications.",
@@ skipping 44 matching lines @@
                     { "name": "documents", "type": "integer" },
                     { "name": "nodes", "type": "integer" },
                     { "name": "jsEventListeners", "type": "integer" }
                 ]
             }
         ]
     },
     {
         "domain": "Page",
         "description": "Actions and events related to the inspected page belong to the page domain.",
+        "capabilities": [
+            {
+                "name": "screencast",
+                "description": "Whether screencast is supported."
+            }
+        ],
         "types": [
             {
                 "id": "ResourceType",
                 "type": "string",
                 "enum": ["Document", "Stylesheet", "Image", "Media", "Font", "Script", "TextTrack", "XHR", "WebSocket", "Other"],
                 "description": "Resource type as it was perceived by the rendering engine."
             },
             {
               "id": "FrameId",
               "type": "string",
@@ skipping 540 matching lines @@
                 "description": "Requests that backend shows scroll bottleneck rects",
                 "parameters": [
                     { "name": "show", "type": "boolean", "description": "True for showing scroll bottleneck rects" }
                 ]
             }
         ]
     },
     {
         "domain": "Emulation",
         "description": "This domain emulates different environments for the page.",
+        "capabilities": [
+            {
+                "name": "deviceMetrics",
+                "description": "Whether device metrics emulation is supported."
+            }
+        ],
         "hidden": true,
         "types": [
             {
                 "id": "Viewport",
                 "type": "object",
                 "description": "Visible page viewport",
                 "properties": [
                     { "name": "scrollX", "type": "number", "description": "X scroll offset in CSS pixels." },
                     { "name": "scrollY", "type": "number", "description": "Y scroll offset in CSS pixels." },
                     { "name": "contentsWidth", "type": "number", "description": "Contents width in CSS pixels." },
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                 "description": "The security state of the page changed.",
                 "parameters": [
                     { "name": "securityState", "$ref": "SecurityState", "description": "Security state." },
                     { "name": "explanations", "type": "array", "items": { "$ref": "SecurityStateExplanation" }, "description": "List of explanations for the security state. If the overall security state is `insecure` or `warning`, at least one corresponding explanation should be included.", "optional": true }
                 ],
                 "handlers": ["browser"]
             }
         ]
     }]
 }