Show message to indicate that mirriring with more than 3 display is not supported.

ash/ash_chromeos_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- ChromeOS-specific strings (included from ash_strings.grd).
      Everything in this file is wrapped in <if expr="chromeos">. -->
 <grit-part>
 
   <!-- Device types -->
   <message name="IDS_ASH_CHROMEBASE" desc="The device name for a Chromebase (versus say Chromebox)">
     Chromebase
   </message>
   <message name="IDS_ASH_CHROMEBIT" desc="The device name for a Chromebit (versus say Chromebox)">
@@ skipping 421 matching lines @@
   </message>
   <message name="IDS_ASH_VIRTUAL_KEYBOARD_TRAY_ACCESSIBLE_NAME" desc="The accessible text for virtual keyboard icon in status tray.">
     Show on-screen keyboard
   </message>
   <message name="IDS_ASH_DISPLAY_FAILURE_ON_MIRRORING" desc="An error message to show that the system failed to enter the mirroring mode.">
     Could not mirror displays since no supported resolutions found. Entered extended desktop instead. Click to send a feedback report.
   </message>
   <message name="IDS_ASH_DISPLAY_FAILURE_ON_NON_MIRRORING" desc="An error message to show that the system failed to enter the extended desktop mode or unknown status. Please translate the parentized text.">
     Dear Monitor, it's not working out between us. (That monitor is not supported. Click to send a feedback report.)
   </message>
+  <message name="IDS_ASH_DISPLAY_MIRRORING_NOT_SUPPORTED" desc="A message used to explain that mirroring with more than two displays is not supported.">
+    Mirroring with more than two displays is not supported.
+  </message>
   <message name="IDS_ASH_DISPLAY_RESOLUTION_CHANGE_ACCEPT" desc="A button label shown in the notification for the resolution change to accept the change">
     Accept
   </message>
   <message name="IDS_ASH_DISPLAY_RESOLUTION_CHANGE_REVERT" desc="A button label shown in the notification for the resolution change to revert the change">
     Revert
   </message>
   <message name="IDS_ASH_DISPLAY_RESOLUTION_TIMEOUT" desc="A message for to notify the user about the timeout of display resolution change.">
     Reverting to old resolution in <ph name="TIMEOUT_SECONDS">$1</ph>
   </message>
   <message name="IDS_ASH_LOW_PERIPHERAL_BATTERY_NOTIFICATION_TEXT" desc="The text of the notification when a peripheral device is in low battery condition.">
@@ skipping 62 matching lines @@
   </message>
   <message name="IDS_DEPRECATED_SHOW_TASK_MANAGER_MSG"
            desc="Notification message to tell users about the deprecation of Shift+ESC shortcut to show the task manager.">
     The shortcut to open the task manager has changed. Please use <ph name="NEW_SHORTCUT">$1<ex>Search+Esc</ex></ph> instead of <ph name="OLD_SHORTCUT">$2<ex>Shift+Esc</ex></ph>.
   </message>
   <message name="IDS_DEPRECATED_NEXT_IME_MSG"
            desc="Notification message to tell users about the deprecation of Shift+Alt shortcut to switch to the next input method.">
     The shortcut to switch to the next input method has changed. Please use <ph name="NEW_SHORTCUT">$1<ex>Ctrl+Shift+Space</ex></ph> instead of <ph name="OLD_SHORTCUT">$2<ex>Shift+Alt</ex></ph>.
   </message>
 </grit-part>