Change zoom presets to be more useful.

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 height of the intersection of two rectangles.
  * @param {Object} rect1 the first rect
  * @param {Object} rect2 the second rect
  * @return {number} the height of the intersection of the rects
  */
@@ skipping 59 matching lines @@
  * 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. This should match the list in
  * components/ui/zoom/page_zoom_constants.h
  */
-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];
+Viewport.ZOOM_FACTORS = [0.25, 1 / 3, 0.5, 2 / 3, 0.75, 0.8, 0.9,
+                         1, 1.1, 1.25, 1.5, 1.75, 2, 2.5, 3, 4, 5];
 
 /**
  * The minimum and maximum range to be used to clip zoom factor.
  */
 Viewport.ZOOM_FACTOR_RANGE = {
   min: Viewport.ZOOM_FACTORS[0],
   max: Viewport.ZOOM_FACTORS[Viewport.ZOOM_FACTORS.length - 1]
 };
 
 /**
@@ skipping 497 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_
     };
   }
 };