Improve robustness of pageCapture.saveAsMHTML custom binding

chrome/renderer/resources/extensions/page_capture_custom_bindings.js

 // Copyright (c) 2012 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.
 
 // Custom binding for the pageCapture API.
 
 var binding = require('binding').Binding.create('pageCapture');
 
 var pageCaptureNatives = requireNative('page_capture');
 var CreateBlob = pageCaptureNatives.CreateBlob;
 var SendResponseAck = pageCaptureNatives.SendResponseAck;
 
 binding.registerCustomHook(function(bindingsAPI) {
   var apiFunctions = bindingsAPI.apiFunctions;
 
   apiFunctions.setCustomCallback('saveAsMHTML',
                                  function(name, request, response) {
-    var path = response.mhtmlFilePath;
-    var size = response.mhtmlFileLength;
+    var callback = request.callback;
+    request.callback = null;
+    if (response) {

[Devlin, 2014/07/10 20:53:28]

nit: inline?

if (response)
  response = CreateBlob(response.mthmlFilePath, response.mhtmlFileLength);

or even

response = response ?
    CreateBlob(response.mthmlFilePath, response.mhtmlFileLength) :
    undefined;

[robwu, 2014/07/10 21:15:56]

Done.

+      var path = response.mhtmlFilePath;
+      var size = response.mhtmlFileLength;
+      response = CreateBlob(path, size);
+    }
 
-    if (request.callback)
-      request.callback(CreateBlob(path, size));
-    request.callback = null;
-
-    // Notify the browser. Now that the blob is referenced from JavaScript,
-    // the browser can drop its reference to it.
-    SendResponseAck(request.id);
+    try {
+      callback(response);
+    } finally {

[Devlin, 2014/07/10 20:53:28]

We usually put a catch here and log the error so the developer can see what went
wrong, e.g.:
https://code.google.com/p/chromium/codesearch#chromium/src/chrome/renderer/re...

[robwu, 2014/07/10 21:15:56]

Done.

+      // Notify the browser. Now that the blob is referenced from JavaScript,
+      // the browser can drop its reference to it.
+      SendResponseAck(request.id);
+    }
   });
 });
 
 exports.binding = binding.generate();