Update external protocol handler strings to use "can't" instead of "cannot".

chrome/app/google_chrome_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 <!-- This file contains definitions of strings that are specific to
 Google Chrome. These strings will be translated for each locale that Google
 Chrome supports. If you update this file, be sure also to update
 chromium_strings.grd. -->
 
 <grit base_dir="." latest_public_release="0" current_release="1"
       output_all_resource_defines="false" source_lang_id="en" enc_check="möl">
   <outputs>
     <output filename="grit/google_chrome_strings.h" type="rc_header">
@@ skipping 304 matching lines @@
         </message>
       </if>
       <if expr="not chromeos and not is_ios">
         <message name="IDS_NEW_TAB_CHROME_WELCOME_PAGE_TITLE"
                  desc="Title for the hard-coded thumbnail that represents the Google Chrome OS Welcome page. This is used on the NTP when there aren't enough thumbnails to show.">
           Welcome to Google Chrome
         </message>
       </if>
       <if expr="chromeos">
         <message name="IDS_EXTERNAL_PROTOCOL_TITLE" desc="External Protocol Dialog Title">
-          Google Chrome OS cannot open this page.
+          Google Chrome OS can't open this page.
         </message>
       </if>
       <!-- Strings used to warn that an OS is not supported -->
       <if expr="is_win">
         <message name="IDS_UNSUPPORTED_OS_PRE_WIN_XP" desc="The text used to warn the user that Windows pre-XP is not supported">
           Google Chrome requires Windows XP or later. Some features may not work.
         </message>
       </if>
       <if expr="not chromeos">
         <message name="IDS_OPTIONS_RELAUNCH_REQUIRED" desc="The message displayed for an option that requires a relaunch to take effect. This appears in a message box if an option is changed that requires a relaunch.">
@@ skipping 954 matching lines @@
           Pin Chrome to your taskbar
         </message>
         <message name="IDS_WIN10_WELCOME_SELECT" desc="An instruction asking the user to select Chrome from an external list of apps.">
           Select <ph name="SMALL_PRODUCT_LOGO">&lt;div class="logo-small"&gt;&lt;/div&gt;</ph> <ph name="BEGIN_BOLD">&lt;strong&gt;</ph>Google Chrome<ph name="END_BOLD">&lt;/strong&gt;</ph>
         </message>
       </if>
 
     </messages>
   </release>
 </grit>