Print Preview: Set page index as pageIndex when pagesPerSheet > 1.

When sending URLs to the PDF plugin, the page index in the URL
needs to be the page index for the displayed PDF document.
When N-up > 1, this will not match the page index from the selected
pages in the UI, since the N-up page numbers will be recalculated
using the number of pages selected and pagesPerSheet Value.  The
new number of pages will be ceil(#pages selected/pagesPerSheet),
and the index of the N-up pages will start from 0.  Hence changing
the function onPagePreviewReady_ to use the N-upped document index
when pagesPerSheet > 1.

In printing, 2-up, 3-up, or more generally N-up refers to a page
layout strategy in which multiple pages are composited onto a
single page.

Bug: 775999
Change-Id: I1488431713815b573eb2798e23b011417b9fa46d
Reviewed-on: https://chromium-review.googlesource.com/1155906
Commit-Queue: Shirleen Lou <xlou@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579588}

chrome/browser/resources/print_preview/new/preview_area.js

@@ -480,7 +480,13 @@ Polymer({
     if (this.inFlightRequestId_ != previewResponseId)
       return;
     const pageNumber = pageIndex + 1;
-    const index = this.getSettingValue('pages').indexOf(pageNumber);
+    let index = this.getSettingValue('pages').indexOf(pageNumber);
+    // When pagesPerSheet > 1, the backend will always return page indices 0 to
+    // N-1, where N is the total page count of the N-upped document.
+    const pagesPerSheet =
+        /** @type {number} */ (this.getSettingValue('pagesPerSheet'));
+    if (pagesPerSheet > 1)
+      index = pageIndex;
     if (index == 0)
       this.onPreviewStart_(previewUid, pageIndex);
     if (index != -1)