Functionality has been requested in the Extension API for Javascript to...

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "types": [
       {
         "id": "Port",
         "type": "object",
         "properties": {
           "name": {"type": "string"},
           "onDisconnect": {"type": "object"},
@@ skipping 380 matching lines @@
         "parameters": [
           {"type": "integer", "name": "tabId", "minimum": 0, "optional": true},
           {
             "type": "function",
             "name": "callback",
             "parameters": [
               {"type": "string", "name": "language"}
             ]
           }
         ]
+      },
+      {
+        "name": "captureVisibleTab",
+        "type": "function",
+        "description": "Captures the visible area of the visible tab in the given window.",
+        "parameters": [
+          {"type": "integer", "name": "windowId", "minimum": 0, "optional": true, "description": "The target window. If <var>null</var> or <var>undefined</var>, the 'current' window will be assumed."},
+          {"type": "function", "name": "callback", "parameters": [
+              {"type": "string", "name": "dataUrl", "description": "a data url encoding of the captured tab."}
+            ]
+          }
+        ]
       }
     ],
     "events": [
       {
         "name": "onCreated",
         "type": "function",
         "description": "",
         "parameters": [
           {"$ref": "Tab", "name": "tab"}
         ]
@@ skipping 394 matching lines @@
         "description": "",
         "parameters": [
           {"type": "string", "name": "url", "optional": true},
           {"type": "function", "name": "callback", "optional": true, "parameters": []}
         ]
       }
     ],
     "events": []
   }
 ]