Animations: Allow document animation timelines to have a playback rate

Source/devtools/protocol.json

 {
     "version": { "major": "1", "minor": "1" },
     "domains": [{
         "domain": "Inspector",
         "hidden": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications."
@@ skipping 565 matching lines @@
                 "handlers": ["browser"]
             },
             {
                 "name": "setColorPickerEnabled",
                 "parameters": [
                     { "name": "enabled", "type": "boolean", "description": "Shows / hides color picker" }
                 ],
                 "description": "Shows / hides color picker",
                 "hidden": true,
                 "handlers": ["browser"]
+            },
+            {
+                "name": "setAnimationsPlaybackRate",
+                "parameters": [
+                    { "name": "playbackRate", "type": "number", "description": "Playback rate for animations on page" }
+                ],
+                "description": "Sets the playback rate of the document timeline.",
+                "hidden": true
             }
         ],
         "events": [
             {
                 "name": "domContentEventFired",
                 "parameters": [
                     { "name": "timestamp", "type": "number" }
                 ]
             },
             {
@@ skipping 3938 matching lines @@
                     { "name": "isRunning", "type": "boolean", "description": "If the animation player is still running." }
                 ],
                 "description": "Gets the state of an AnimationPlayer.",
                 "hidden": true
             }
         ],
         "events": [
         ]
     }]
 }