Added a test that currently is able to print to pdf.

chrome/browser/resources/print_preview/print_preview.js

Index: chrome/browser/resources/print_preview/print_preview.js
diff --git a/chrome/browser/resources/print_preview/print_preview.js b/chrome/browser/resources/print_preview/print_preview.js
index ac3a0ab789135db2746d6fe3b522398b759867d9..b49c4b246107b8006f3b0718227c2d28f85d1a35 100644
--- a/chrome/browser/resources/print_preview/print_preview.js
+++ b/chrome/browser/resources/print_preview/print_preview.js
@@ -1040,4 +1040,25 @@ cr.define('print_preview', function() {
 window.addEventListener('DOMContentLoaded', function() {
   printPreview = new print_preview.PrintPreview();
   printPreview.initialize();
+
+  function temp() {
+    chrome.send('UILoaded');
+  }
+  (function func() {
+    var settings = $('layout-settings');
+    if (settings.offsetWidth > 10) {

[Lei Zhang, 2014/06/13 22:20:11]

So instead of looping and look for this as the signal that the print preview is
reading, what if you changed onInitialSettingsSet_() to do a chrome.send() to
indicate it's ready. Then the C++ test can call a function in the JS file here
to click() the DOM elements. Also add a chrome.send() call to
onPreviewGenerationDone_() to send another message and listen for that on the
C++ side?

+      if (printPreview) {
+        printPreview.destinationStore_.selectDefaultDestination_();
+        var portrait = document.getElementsByClassName(
+            'layout-settings-landscape-radio')[0];
+        if (portrait) {
+          portrait.click();
+        }
+        setTimeout(temp, 6000);
+      }
+    }
+    else {
+      setTimeout(func, 50);
+    }
+  })();
 });