[CrOS Settings] Move OS about page strings from settings strings

This CL moves chromeOS strings for settings about page from
settings_strings.grdp to os_settings_strings.grdp. It continues
the process of moving all chromeOS specific strings.

Bug: 967883
Change-Id: Ibb1306490bd1bce9165d618e2c52c8dc783ae8be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2070378
Commit-Queue: Nnamdi Theodore Johnson-kanu <tjohnsonkanu@google.com>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745622}

chrome/app/shared_settings_strings.grdp

@@ -248,4 +248,9 @@
   <message name="IDS_SETTINGS_SYNC_SETUP_CANCEL_DIALOG_BODY" desc="The body text of the dialog to confirm the sync cancellation.">
     You can turn on sync anytime in settings
   </message>
+
+  <!-- About Subpage-->
+  <message name="IDS_SETTINGS_ABOUT_UPGRADE_CHECK_STARTED" desc="Status label: About to start checking for updates">
+    Checking for updates
+  </message>
 </grit-part>