Add message for tab sharing infobars for sharing untitled tabs.

When sharing a tab that does not have a URL, host or tab title, the tab
sharing infobar should simple display "Sharing a tab". Add message
resource for this case.

Bug: 1011379
Change-Id: Ibe67b32f3eb55695067c534552559cec8b0f19e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1841851Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Marina Ciocea <marinaciocea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702932}

chrome/app/generated_resources.grd

@@ -3131,6 +3131,9 @@ are declared in tools/grit/grit_rule.gni.
       <message name="IDS_TAB_SHARING_INFOBAR_SHARING_CURRENT_TAB_LABEL" desc="Text displayed on an infobar when current tab is being shared.">
        Sharing this tab to <ph name="APP_NAME">$1<ex>meet.google.com</ex></ph>
       </message>
+      <message name="IDS_TAB_SHARING_INFOBAR_SHARING_ANOTHER_UNTITLED_TAB_LABEL" desc="Text displayed on an infobar when another (non-current) untitled tab is being shared.">
+       Sharing a tab to <ph name="APP_NAME">$1<ex>meet.google.com</ex></ph>
+      </message>
       <message name="IDS_TAB_SHARING_INFOBAR_SHARING_ANOTHER_TAB_LABEL" desc="Text displayed on an infobar when another (non-current) tab is being shared.">
        Sharing <ph name="TAB_NAME">$1<ex>youtube.com</ex></ph> to <ph name="APP_NAME">$2<ex>meet.google.com</ex></ph>
       </message>