Change zoom presets to be more useful.

chrome/browser/resources/pdf/viewport.js

Index: chrome/browser/resources/pdf/viewport.js
diff --git a/chrome/browser/resources/pdf/viewport.js b/chrome/browser/resources/pdf/viewport.js
index 4cb8fee0f66a994dd1959295b0ab8483d191890b..eb4623d6e594b15d4d2ee3d22af71160e294d110 100644
--- a/chrome/browser/resources/pdf/viewport.js
+++ b/chrome/browser/resources/pdf/viewport.js
@@ -77,8 +77,8 @@ Viewport.SCROLL_INCREMENT = 40;
  * 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.