Remove support for the keygen tag

content/app/strings/content_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 
 <!-- This file contains definitions of resources that will be translated for
 each locale. Specifically, these are UI strings that are used by content that
 need to be translated for each locale.-->
 
 <!-- Some of these strings and string descriptions were taken from
 WebKit/win/WebCoreLocalizedStrings.cpp so we include the original license
 below:
 
@@ skipping 710 matching lines @@
       </message>
 
       <message name="IDS_AX_WEEK_OF_YEAR_FIELD_TEXT" desc="Accessible description of the week of year field in a date/time control">
         Week
       </message>
 
       <message name="IDS_AX_YEAR_FIELD_TEXT" desc="Accessible description of the year field in a date/time control">
         Year
       </message>
 
-      <message name="IDS_KEYGEN_HIGH_GRADE_KEY" desc="High-grade cryptographic key size menu item">
-        2048 (High Grade)
-      </message>
-      <message name="IDS_KEYGEN_MED_GRADE_KEY" desc="Medium-grade cryptographic key size menu item">
-        1024 (Medium Grade)
-      </message>
-
       <message name="IDS_FORM_INPUT_WEEK_TEMPLATE" desc="A specfic week (1-53) in a specific year shown in a form control">
         Week <ph name="WEEKNUMBER">$2<ex>51</ex></ph>, <ph name="YEAR">$1<ex>2012</ex></ph>
       </message>
       <message name="IDS_FORM_VALIDATION_VALUE_MISSING_MULTIPLE_FILE" desc="Heading or short sentence shown when a file upload control in a webpage requires one or more files selected, but the user didn't specify any files.">
         Please select one or more files.
       </message>
       <message name="IDS_FORM_VALIDATION_TYPE_MISMATCH" desc="Heading or short sentence shown when a form control in a webpage requires specific type such as email address or URL, but the specified value does not comform to the type.">
         Invalid value.
       </message>
       <message name="IDS_FORM_VALIDATION_TYPE_MISMATCH_EMAIL_EMPTY" desc="Heading or short sentence shown when there is an email field in a form and a user specified an empty value.">
@@ skipping 154 matching lines @@
         Off
       </message>
 
       <message name="IDS_PLUGIN_INITIALIZATION_ERROR" desc="A message displayed when a plugin failed to load">
         Couldn't load plugin.
       </message>
 
     </messages>
   </release>
 </grit>