DevTools: expose layer compositing reasons and paint count via layer tree agent

Source/devtools/protocol.json

 {
     "version": { "major": "1", "minor": "0" },
     "domains": [{
         "domain": "Inspector",
         "hidden": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications."
@@ skipping 3832 matching lines @@
             },
             {
                 "name": "getLayers",
                 "parameters": [
                     { "name": "nodeId", "optional": true, "$ref": "DOM.NodeId", "description": "Root of the subtree for which we want to gather layers (return entire tree if not specified)" }
                 ],
                 "description": "Returns the layer tree structure of the current page.",
                 "returns": [
                     { "name": "layers", "type": "array", "items": { "$ref": "Layer" }, "description": "Child layers." }
                 ]
+            },
+            {
+                "name": "compositingReasons",
+                "parameters": [
+                    { "name": "layerId", "$ref": "LayerId", "description": "The id of the layer for which we want to get the reasons it was composited." }
+                ],
+                "description": "Provides the reasons why the given layer was composited.",
+                "returns": [
+                    { "name": "compositingReasons", "type": "array", "items": { "type": "string" }, "description": "A list of strings specifying reasons for the given layer to become composited." }
+                ]
             }
         ],
         "events": [
             {
                 "name": "layerTreeDidChange"
             }
         ]
     },
     {
         "domain": "Tracing",
@@ skipping 17 matching lines @@
                 "parameters": [
                     { "name": "value", "type": "array", "items": { "type": "object" } }
                 ]
             },
             {
                 "name": "tracingComplete"
             }
         ]
     }]
 }