dark_mode: Extend dark theme character limit.

Bug: b/168511137
Change-Id: I31f4a80fe543b8cc02dfe14b2d378ded6951e4ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2411703Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807079}

ash/ash_strings.grd

@@ -330,7 +330,7 @@ This file contains the strings for ash.
       <message name="IDS_ASH_STATUS_TRAY_CAST_CAST_DESKTOP" desc="The label used in the tray popup to tell the user we are casting the desktop.">
         Casting screen to <ph name="RECEIVER_NAME">$1<ex>Living Room</ex></ph>
       </message>
-      <message name="IDS_ASH_STATUS_TRAY_DARK_THEME" desc="The label used as the header in the dark theme popup. [CHAR_LIMIT=17">
+      <message name="IDS_ASH_STATUS_TRAY_DARK_THEME" desc="The label used as the header in the dark theme popup. [CHAR_LIMIT=19">
         Dark theme
       </message>
       <message name="IDS_ASH_STATUS_TRAY_DARK_THEME_TOGGLE_TOOLTIP" desc="The tooltip text used for the button in the status tray to toggle the Dark theme feature on or off.">