[NaCl SDK] Screen capture extension for use with SDK visual testing.

native_client_sdk/src/build_tools/screenshot_extension/background.js

+// Copyright (c) 2013 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.
+
+function takeScreenshot(onScreenshot) {
+  console.log('Taking screenshot.');
+  chrome.tabs.captureVisibleTab(null, {format: 'png'}, function(img) {
+    console.log('Got screenshot, returning...');
+    onScreenshot(img);
+  });
+}
+
+chrome.tabs.onUpdated.addListener(function(tabId, changeInfo, tab) {
+  if (changeInfo.status != 'complete')
+    return;
+
+  chrome.tabs.executeScript(tabId,
+                            {file: 'injected.js', runAt: 'document_start'});
+});
+
+chrome.runtime.onMessage.addListener(
+    function(request, sender, sendResponse) {
+      takeScreenshot(sendResponse);
+
+      // Keep the sendResponse channel open, so a response can be sent
+      // asynchronously.
+      return true;
+    });