DevTools: check for canForceCompositingMode before trying to emulate device metrics.

Source/devtools/protocol.json

 {
     "version": { "major": "1", "minor": "1" },
     "domains": [{
         "domain": "Inspector",
         "hidden": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications."
@@ skipping 464 matching lines @@
             {
                 "name": "setShowViewportSizeOnResize",
                 "description": "Paints viewport size upon main frame resize.",
                 "parameters": [
                     { "name": "show", "type": "boolean", "description": "Whether to paint size or not." },
                     { "name": "showGrid", "type": "boolean", "optional": true, "description": "Whether to paint grid as well." }
                 ],
                 "hidden": true
             },
             {
+                "name": "canForceCompositingMode",
+                "description": "Returns whether page can be forced into compositing mode.",
+                "returns": [
+                    { "name": "result", "type": "boolean", "description": "Whether page can be forced into compositing mode." }
+                ],
+                "hidden": true
+            },
+            {
                 "name": "setForceCompositingMode",
                 "description": "Force accelerated compositing mode for inspected page.",
                 "parameters": [
                     { "name": "force", "type": "boolean", "description": "Whether to force accelerated compositing or not." }
                 ],
                 "hidden": true
             }
         ],
         "events": [
             {
@@ skipping 3540 matching lines @@
                 "parameters": [
                     { "name": "value", "type": "array", "items": { "type": "object" } }
                 ]
             },
             {
                 "name": "tracingComplete"
             }
         ]
     }]
 }