Add string for the LTS build

Add a text to be displayed in Chrome OS settings to tell the user that
they are currently on the LTS channel.

This CL only adds the new string without implementing its usage in order
to have it translated ASAP. The actual usage of the new string will be
added with https://crrev.com/c/2355925.

Bug: 1105495
Change-Id: I055f1bff4aeb5aef2e75a3f0f6bec089c3a6abaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2323292
Commit-Queue: Jana Grill <janagrill@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Victor-Gabriel Savu <vsavu@google.com>
Cr-Commit-Position: refs/heads/master@{#799121}

chrome/app/os_settings_strings.grdp

@@ -123,7 +123,7 @@
   <message name="IDS_SETTINGS_ABOUT_PAGE_UPDATE_WARNING_TITLE" desc="The title of the warning about update over mobile data.">
     Download update using mobile data
   </message>
-    <message name="IDS_SETTINGS_ABOUT_PAGE_CURRENT_CHANNEL_BETA" desc="The beta label in the message about current channel.">
+  <message name="IDS_SETTINGS_ABOUT_PAGE_CURRENT_CHANNEL_BETA" desc="The beta label in the message about current channel.">
     beta
   </message>
   <message name="IDS_SETTINGS_ABOUT_PAGE_CURRENT_CHANNEL_CANARY" desc="The canary label in the message about current channel.">
@@ -138,6 +138,9 @@
   <message name="IDS_SETTINGS_ABOUT_PAGE_CURRENT_CHANNEL_STABLE" desc="The stable label in the message about current channel.">
     stable
   </message>
+  <message name="IDS_SETTINGS_ABOUT_PAGE_CURRENT_CHANNEL_STABLE_TT" desc="The stable (trusted tester) label in the message about current channel.">
+    stable (trusted tester)
+  </message>
   <message name="IDS_SETTINGS_ABOUT_PAGE_CHECK_FOR_UPDATES" desc="The button label to check for updates and apply (download and install) if found">
     Check for updates
   </message>

chrome/app/os_settings_strings_grdp/IDS_SETTINGS_ABOUT_PAGE_CURRENT_CHANNEL_STABLE_TT.png.sha1

+5b752193cea0bfbf9256a2e238f92f9569e4f6ff
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