Re-land alexbost's experimental offscreenTabs API.

chrome/common/extensions/api/tabs.json

 [
   {
     "namespace": "tabs",
     "dependencies": [ "extension", "windows" ],
     "types": [
       {
         "id": "Tab",
         "type": "object",
         "properties": {
           "id": {"type": "integer", "minimum": 0, "description": "The ID of the tab. Tab IDs are unique within a browser session."},
@@ skipping 513 matching lines @@
             "description": "Set parameters of image capture, such as the format of the resulting image.",
             "properties": {
               "format": {
                 "type": "string",
                 "optional": true,
                 "enum": ["jpeg", "png"],
                 "description": "The format of the resulting image.  Default is jpeg."
               },
               "quality": {
                 "type": "integer",
-                "name": "quality",
                 "optional": true,
                 "minimum": 0,
                 "maximum": 100,
                 "description": "When format is 'jpeg', controls the quality of the resulting image.  This value is ignored for PNG images.  As quality is decreased, the resulting image will have more visual artifacts, and the number of bytes needed to store it will decrease."
               }
             }
           },
           {
             "type": "function", "name": "callback", "parameters": [
               {"type": "string", "name": "dataUrl", "description": "A data URL which encodes an image of the visible area of the captured tab. May be assigned to the 'src' property of an HTML Image element for display."}
@@ skipping 282 matching lines @@
             "name": "removeInfo",
             "properties": {
               "isWindowClosing": {"type": "boolean", "description": "True when the tab is being closed because its window is being closed." }
             }
           }
         ]
       }
     ]
   }
 ]