Use custom scrollbars for print preview on non-retina mac displays.

chrome/browser/resources/pdf/pdf_scripting_api.js

Index: chrome/browser/resources/pdf/pdf_scripting_api.js
diff --git a/chrome/browser/resources/pdf/pdf_scripting_api.js b/chrome/browser/resources/pdf/pdf_scripting_api.js
index 84b94b12450f611cde0e95e394438f6b15a4b465..fa570aeeb989b0d5f62c29660ceaf7cadb3d92a5 100644
--- a/chrome/browser/resources/pdf/pdf_scripting_api.js
+++ b/chrome/browser/resources/pdf/pdf_scripting_api.js
@@ -250,6 +250,8 @@ PDFScriptingAPI.prototype = {
   },
 };
 
+var IS_MAC_PARAM = 'isMac&';
+
 /**
  * Creates a PDF viewer with a scripting interface. This is basically 1) an
  * iframe which is navigated to the PDF viewer extension and 2) a scripting
@@ -260,9 +262,11 @@ PDFScriptingAPI.prototype = {
  */
 function PDFCreateOutOfProcessPlugin(src) {
   var iframe = window.document.createElement('iframe');
+  var isMac = cr.isMac ? IS_MAC_PARAM : '';
   iframe.setAttribute(
       'src',
-      'chrome-extension://mhjfbmdgcfjbbpaeojofohoefgiehjai/index.html?' + src);
+      'chrome-extension://mhjfbmdgcfjbbpaeojofohoefgiehjai/index.html?' +
+      isMac + src);
   // Prevent the frame from being tab-focusable.
   iframe.setAttribute('tabindex', '-1');
   var client = new PDFScriptingAPI(window);