Supply accessibility description for refine button.

This change adds accessibility content description for refine button

Bug: 939014
Change-Id: Ief2f5d1e5aa14498d03eda7827788dfd0d752ea6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506557
Commit-Queue: Ender <ender@google.com>
Auto-Submit: Ender <ender@google.com>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638624}

chrome/android/java/res/layout/omnibox_answer_suggestion.xml

@@ -60,7 +60,7 @@
     <ImageView
         android:background="?attr/selectableItemBackground"
         android:clickable="true"
-        android:contentDescription="@null"
+        android:contentDescription="@string/accessibility_omnibox_btn_refine"
         android:focusable="true"
         android:id="@id/omnibox_answer_refine_icon"
         android:layout_alignBottom="@id/omnibox_answer"