Instead of ArrayBuffer, pass blob with printerProvider.onPrintRequested

extensions/renderer/resources/printer_provider_custom_bindings.js

 // Copyright 2015 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.
 
 var binding = require('binding').Binding.create('printerProvider');
 var printerProviderInternal = require('binding').Binding.create(
     'printerProviderInternal').generate();
 var eventBindings = require('event_bindings');
+var blobNatives = requireNative('blob_natives');
 
 var printerProviderSchema =
     requireNative('schema_registry').GetSchema('printerProvider')
 var utils = require('utils');
 var validate = require('schemaUtils').validate;
 
 // Custom bindings for chrome.printerProvider API.
 // The bindings are used to implement callbacks for the API events. Internally
 // each event is passed requestId argument used to identify the callback
 // associated with the event. This argument is massaged out from the event
 // arguments before dispatching the event to consumers. A callback is appended
 // to the event arguments. The callback wraps an appropriate
 // chrome.printerProviderInternal API function that is used to report the event
 // result from the extension. The function is passed requestId and values
 // provided by the extension. It validates that the values provided by the
 // extension match chrome.printerProvider event callback schemas. It also
 // ensures that a callback is run at most once. In case there is an exception
 // during event dispatching, the chrome.printerProviderInternal function
 // is called with a default error value.
 //
 
 // Handles a chrome.printerProvider event as described in the file comment.
 // |eventName|: The event name.
+// |prepareArgsForDispatch|: Function called before dispatching the event to
+//     the extension. It's called with original event |args| list and callback
+//     that should be called when the |args| are ready for dispatch. The
+//     callbacks should report whether the argument preparation was successful.
+//     The function should not change the first argument, which contains the
+//     request id.
 // |resultreporter|: The function that should be called to report event result.
-//                   One of chrome.printerProviderInternal API functions.
-function handleEvent(eventName, resultReporter) {
+//     One of chrome.printerProviderInternal API functions.
+function handleEvent(eventName, prepareArgsForDispatch, resultReporter) {
   eventBindings.registerArgumentMassager(
       'printerProvider.' + eventName,
       function(args, dispatch) {
         var responded = false;
 
         // Validates that the result passed by the extension to the event
         // callback matches the callback schema. Throws an exception in case of
         // an error.
         var validateResult = function(result) {
           var eventSchema =
@@ skipping 17 matching lines @@
           var finalResult = null;
           try {
             validateResult(result);  // throws on failure
             finalResult = result;
           } finally {
             responded = true;
             resultReporter(args[0] /* requestId */, finalResult);
           }
         };
 
-        dispatch(args.slice(1).concat(reportResult));
+        prepareArgsForDispatch(args, function(success) {
+          if (!success) {
+            // Do not throw an exception since the extension should not yet be
+            // aware of the event.
+            resultReporter(args[0], null);

[not at google - send to devlin, 2015/03/04 21:54:49]

document that args[0] is the request ID, like you have above.

[tbarzic, 2015/03/04 23:24:34]

Done.

+            return;
+          }
+          dispatch(args.slice(1).concat(reportResult));
+        });
       });
 }
 
-handleEvent('onGetPrintersRequested', printerProviderInternal.reportPrinters);
+// Sets up printJob.document property for a print request. The printJob argument
+// is at |args[1]|.
+function createPrintRequestBlobArguments(args, callback) {
+  printerProviderInternal.getPrintData(args[0], function(blobInfo) {

[not at google - send to devlin, 2015/03/04 21:54:49]

and here

[tbarzic, 2015/03/04 23:24:34]

Done.

+    if (chrome.runtime.lastError) {
+      callback(false);
+      return;
+    }
+
+    args[1].document = blobNatives.TakeBrowserProcessBlob(

[not at google - send to devlin, 2015/03/04 21:54:49]

and document what args[1] is.

[tbarzic, 2015/03/04 23:24:34]

Done.

+        blobInfo.blobUuid, blobInfo.type, blobInfo.size);
+    callback(true);
+  });
+}
+
+handleEvent('onGetPrintersRequested',
+            function(args, callback) { callback(true); },
+            printerProviderInternal.reportPrinters);
 
 handleEvent('onGetCapabilityRequested',
+            function(args, callback) { callback(true); },
             printerProviderInternal.reportPrinterCapability);
 
-handleEvent('onPrintRequested', printerProviderInternal.reportPrintResult);
+handleEvent('onPrintRequested',
+            createPrintRequestBlobArguments,
+            printerProviderInternal.reportPrintResult);
 
 exports.binding = binding.generate();