Change the default font size in ChromeOS from 12px to 13px.

ui/strings/app_locale_settings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 <grit latest_public_release="0" current_release="1">
   <outputs>
     <output filename="grit/app_locale_settings.h" type="rc_header">
       <emit emit_type='prepend'></emit>
     </output>
     <output filename="app_locale_settings_am.pak" type="data_package" lang="am" />
     <output filename="app_locale_settings_ar.pak" type="data_package" lang="ar" />
     <if expr="use_third_party_translations">
       <output filename="app_locale_settings_ast.pak" type="data_package" lang="ast" />
@@ skipping 238 matching lines @@
 
         <!-- The relative font size in % used in Web UI (e.g. History).  -->
         <message name="IDS_WEB_FONT_SIZE" use_name_for_id="true">
           75%
         </message>
       </if>
       <!-- For Chrome OS -->
       <if expr="chromeos">
         <!-- The font name like: 'Font Name, 10' -->
         <message name="IDS_UI_FONT_FAMILY_CROS" use_name_for_id="true">
-          Noto Sans UI, 12px
+          Noto Sans UI, 13px
         </message>
 
         <!-- The font used in Web UI (e.g. History). -->
         <message name="IDS_WEB_FONT_FAMILY" use_name_for_id="true">
           'Noto Sans UI', sans-serif
         </message>
 
         <!-- The relative font size in % used in Web UI (e.g. History).  -->
         <message name="IDS_WEB_FONT_SIZE" use_name_for_id="true">
           75%
         </message>
       </if>
     </messages>
   </release>
 </grit>