Implement webview.captureVisibleRegion()

extensions/common/api/web_view_internal.json

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
   {
     "namespace": "webViewInternal",
     "description": "none",
     "compiler_options": {
       "implemented_in": "extensions/browser/api/guest_view/web_view/web_view_internal_api.h"
@@ skipping 652 matching lines @@
         "name": "terminate",
         "type": "function",
         "parameters": [
           {
             "type": "integer",
             "name": "instanceId"
           }
         ]
       },
       {
+        "name": "captureVisibleRegion",
+        "type": "function",
+        "description": "foo",
+        "parameters": [
+          {
+            "type": "integer",
+            "name": "instanceId",
+            "description": "The instance ID of the guest <webview> process."
+          },
+          {
+            "$ref": "extensionTypes.ImageDetails",
+            "name": "options",
+            "optional": true
+          },
+          {
+            "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."
+              }
+            ]
+          }
+        ]
+      },
+      {
         "name": "clearData",
         "type": "function",
         "description": "Clears various types of browsing data stored in a storage partition of a <webview>.",
         "parameters": [
           {
             "type": "integer",
             "name": "instanceId",
             "description": "The instance ID of the guest <webview> process."
           },
           {
@@ skipping 10 matching lines @@
             "type": "function",
             "description": "Called when deletion has completed.",
             "optional": true,
             "parameters": []
           }
         ]
       }
     ]
   }
 ]