Card unmasking prompt - add accessibility for checkbox tooltip

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

Index: chrome/android/java/res/layout/autofill_card_unmask_prompt.xml
diff --git a/chrome/android/java/res/layout/autofill_card_unmask_prompt.xml b/chrome/android/java/res/layout/autofill_card_unmask_prompt.xml
index d167d77b1f8b9800fde3b4d8228eb792e3b1a97c..a65b132d78374fbff20c80ac77430f0bd2883d7a 100644
--- a/chrome/android/java/res/layout/autofill_card_unmask_prompt.xml
+++ b/chrome/android/java/res/layout/autofill_card_unmask_prompt.xml
@@ -131,7 +131,6 @@
             android:layout_height="wrap_content"
             android:layout_marginTop="16dp">
 
-            <!-- TODO(estade): Content description? -->
             <ImageView
                 android:id="@+id/store_locally_tooltip_icon"
                 android:layout_width="wrap_content"
@@ -141,7 +140,7 @@
                 android:layout_centerVertical="true"
                 android:padding="12dp"
                 android:src="@drawable/help_outline"
-                android:contentDescription="@null" />
+                android:contentDescription="@string/learn_more" />
 
            <CheckBox
                 android:id="@+id/store_locally_checkbox"
@@ -179,7 +178,6 @@
             android:layout_marginBottom="8dp"
             android:visibility="gone" />
 
-        <!--  TODO(estade): add a real content description for this image. -->
         <ImageView
             android:id="@+id/verification_success"
             android:layout_width="wrap_content"