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

chrome/browser/chrome_page_zoom_constants.cc

Index: chrome/browser/chrome_page_zoom_constants.cc
===================================================================
--- chrome/browser/chrome_page_zoom_constants.cc	(revision 290677)
+++ chrome/browser/chrome_page_zoom_constants.cc	(working copy)
@@ -6,6 +6,8 @@
 
 namespace chrome_page_zoom {
 
+// This list is duplicated in chrome/browser/resources/pdf/viewport.js. Please
+// make sure the two match.
 const double kPresetZoomFactors[] = { 0.25, 0.333, 0.5, 0.666, 0.75, 0.9, 1.0,
                                       1.1, 1.25, 1.5, 1.75, 2.0, 2.5, 3.0, 4.0,
                                       5.0 };