DevTools: Load document (html) content from disk cache in page agent enabling.

Source/devtools/protocol.json

 {
     "version": { "major": "1", "minor": "1" },
     "domains": [{
         "domain": "Inspector",
         "hidden": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications."
@@ skipping 279 matching lines @@
             {
                 "name": "getResourceTree",
                 "description": "Returns present frame / resource tree structure.",
                 "returns": [
                     { "name": "frameTree", "$ref": "FrameResourceTree", "description": "Present frame / resource tree structure." }
                 ],
                 "hidden": true
             },
             {
                 "name": "getResourceContent",
+                "async": true,
                 "description": "Returns content of the given resource.",
                 "parameters": [
                     { "name": "frameId", "$ref": "FrameId", "description": "Frame id to get resource for." },
                     { "name": "url", "type": "string", "description": "URL of the resource to get content for." }
                 ],
                 "returns": [
                     { "name": "content", "type": "string", "description": "Resource content." },
                     { "name": "base64Encoded", "type": "boolean", "description": "True, if content was served as base64." }
                 ],
                 "hidden": true
@@ skipping 3879 matching lines @@
             {
                 "name": "dataAvailable",
                 "parameters": [
                     {"name": "value", "type": "array", "items": { "$ref": "PowerEvent" }, "description": "List of power events." }
                 ],
                 "handlers": ["browser", "frontend"]
             }
         ]
     }]
 }