PDF: Add a quick note regarding zoom factors in the OOP JS code.

chrome/browser/resources/pdf/viewport.js

 // Copyright 2014 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.
 
 /**
  * Returns the area of the intersection of two rectangles.
  * @param {Object} rect1 the first rect
  * @param {Object} rect2 the second rect
  * @return {number} the area of the intersection of the rects
  */
@@ skipping 52 matching lines @@
 /**
  * The increment to scroll a page by in pixels when up/down/left/right arrow
  * keys are pressed. Usually we just let the browser handle scrolling on the
  * window when these keys are pressed but in certain cases we need to simulate
  * these events.
  */
 Viewport.SCROLL_INCREMENT = 40;
 
 /**
  * Predefined zoom factors to be used when zooming in/out. These are in
- * ascending order.
+ * ascending order. This should match the list in
+ * chrome/browser/chrome_page_zoom_constants.cc.
  */
 Viewport.ZOOM_FACTORS = [0.25, 0.333, 0.5, 0.666, 0.75, 0.9, 1,
                          1.1, 1.25, 1.5, 1.75, 2, 2.5, 3, 4, 5];
 
 /**
  * The width of the page shadow around pages in pixels.
  */
 Viewport.PAGE_SHADOW = {top: 3, bottom: 7, left: 5, right: 5};
 
 Viewport.prototype = {
@@ skipping 445 matching lines @@
     spaceOnLeft = Math.max(spaceOnLeft, 0);
 
     return {
       x: x * this.zoom_ + spaceOnLeft - this.window_.pageXOffset,
       y: insetDimensions.y * this.zoom_ - this.window_.pageYOffset,
       width: insetDimensions.width * this.zoom_,
       height: insetDimensions.height * this.zoom_
     };
   }
 };