[DevTools] Explicitly enable/disable Page.viewportChanged protocol event.

Source/devtools/protocol.json

 {
     "version": { "major": "1", "minor": "1" },
     "domains": [{
         "domain": "Inspector",
         "hidden": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications."
@@ skipping 361 matching lines @@
                 "name": "clearDeviceMetricsOverride",
                 "description": "Clears the overriden device metrics.",
                 "hidden": true
             },
             {
                 "name": "resetScrollAndPageScaleFactor",
                 "description": "Requests that scroll offsets and page scale factor are reset to initial values.",
                 "hidden": true
             },
             {
+                "name": "setViewportChangedEventEnabled",
+                "hidden": true,
+                "parameters": [
+                    { "name": "enabled", "type": "boolean", "description": "True to enable viewportChanged event, false to disable it." }
+                ],
+                "description": "Enables/disables viewportChanged event."
+            },
+            {
                 "name": "setShowPaintRects",
                 "description": "Requests that backend shows paint rectangles",
                 "parameters": [
                     { "name": "result", "type": "boolean", "description": "True for showing paint rectangles" }
                 ],
                 "hidden": true
             },
             {
                 "name": "setShowDebugBorders",
                 "description": "Requests that backend shows debug borders on layers",
@@ skipping 3846 matching lines @@
             {
                 "name": "dataAvailable",
                 "parameters": [
                     {"name": "value", "type": "array", "items": { "$ref": "PowerEvent" }, "description": "List of power events." }
                 ],
                 "handlers": ["browser", "frontend"]
             }
         ]
     }]
 }