PDF Viewer: Fix edge case at bottom of document

If more than one page was visible in the viewport, getPageAtY_(y) fell
through to returning 0 when y was the bottom of the document, because
in the last iteration of the loop, where max = min = page = N-1 (for an
N page document), the bottom of the page was exactly equal to y, rather
than strictly > y.

This resulted in the most visible page being set to N-1 rather than N
when the document was scrolled to the bottom in this case.

Bug: 1000121
Change-Id: I541204141d1fb7891f1df1bc95d00be28d39bbfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1851086
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704717}

chrome/browser/resources/pdf/viewport.js

@@ -621,7 +621,7 @@ class Viewport {
       const bottom =
           this.pageDimensions_[page].y + this.pageDimensions_[page].height;
 
-      if (top <= y && bottom > y) {
+      if (top <= y && y <= bottom) {
         return page;
       }
 

chrome/test/data/pdf/viewport_test.js

@@ -221,6 +221,11 @@ var tests = [
     mockWindow.scrollTo(0, 170);
     chrome.test.assertEq(1, viewport.getMostVisiblePage());
 
+    // Zoom out so that more than one page fits and scroll to the bottom.
+    viewport.setZoom(0.4);
+    mockWindow.scrollTo(0, 160);
+    chrome.test.assertEq(2, viewport.getMostVisiblePage());
+
     // Zoomed out with the entire document visible.
     viewport.setZoom(0.25);
     mockWindow.scrollTo(0, 0);