Increase Request Mobile Site character limit to 25em.

Per localization request, increasing the character limit of Request
Mobile Site messageto 25em.

BUG=

Review-Url: https://codereview.chromium.org/2797923009
Cr-Commit-Position: refs/heads/master@{#462926}

ios/chrome/app/strings/ios_strings.grd

@@ -1453,7 +1453,7 @@ Your data was encrypted with your sync passphrase on <ph name="TIME">$2<ex>Sept
       <message name="IDS_IOS_TOOLS_MENU_REQUEST_DESKTOP_SITE" desc="The iOS menu item for requesting the desktop site [Length: 15em] [iOS only]">
         Request Desktop Site
       </message>
-      <message name="IDS_IOS_TOOLS_MENU_REQUEST_MOBILE_SITE" desc="The iOS menu item for requesting the mobile site [Length: 15em] [iOS only]">
+      <message name="IDS_IOS_TOOLS_MENU_REQUEST_MOBILE_SITE" desc="The iOS menu item for requesting the mobile site [Length: 25em] [iOS only]">
         Request Mobile Site
       </message>
       <message name="IDS_IOS_TOOLS_MENU_SETTINGS" desc="The iOS menu item for opening the settings [Length: 15em] [iOS only]">