Add "Unknown" to source language dropdown menu.

Create a separate SourceLanguageComboboxModel to add "Unknown" option to combobox dropdown menu. "Unknown" is selected by default when source language is undetected or unavailable.
Bug: 721600

Change-Id: I55228eea20be7ba1d361f5651400411f4e58d977
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1597275
Commit-Queue: Haohao Wang <[email protected]>
Reviewed-by: anthonyvd <[email protected]>
Cr-Commit-Position: refs/heads/master@{#657509}
diff --git a/components/translate_strings.grdp b/components/translate_strings.grdp
index 0a8b94b..73f4ca5 100644
--- a/components/translate_strings.grdp
+++ b/components/translate_strings.grdp
@@ -88,4 +88,9 @@
     Undo
   </message>
 
+  <!-- Translate Combobox Text -->
+  <message name="IDS_TRANSLATE_UNKNOWN_SOURCE_LANGUAGE" desc="This text is added to source language combobox dropdown menu for source language not supported or detected.">
+    Unknown
+  </message>
+
 </grit-part>