Adding pref to store the user-selected proximity threshold.

chrome/app/settings_strings.grdp

Index: chrome/app/settings_strings.grdp
diff --git a/chrome/app/settings_strings.grdp b/chrome/app/settings_strings.grdp
index bcc5c414d31ef420b2f3a18370cfb6cb3840fc2a..37aebd2420397d8fa5232f5f3d544c710e3db63b 100644
--- a/chrome/app/settings_strings.grdp
+++ b/chrome/app/settings_strings.grdp
@@ -1106,6 +1106,21 @@
     <message name="IDS_SETTINGS_EASY_UNLOCK_TURN_OFF_RETRY" desc="The label text of the retry button on the Easy unlock turn off dialog when Easy unlock could not be turned off because there is a server error.">
       Retry
     </message>
+    <message name="IDS_SETTINGS_EASY_UNLOCK_PROXIMITY_THRESHOLD_LABEL" desc="The text on the Easy unlock settings page indicating how close the phone should be for unlock to work.">
+      Only unlock this Chromebook when your phone is

[stevenjb, 2017/07/17 16:43:36]

I'm concerned about how well this will localize. Generally a dropdown would have
a label and a list of self contained values (vs. completing a sentence, which
works in English but not as well in other languages). 

e.g. "Phone distance to unlock this Chromebook"

[sacomoto, 2017/07/20 19:50:15]

This was changed to "Distance needed for phone to unlock this Chromebook"

+    </message>
+    <message name="IDS_SETTINGS_EASY_UNLOCK_PROXIMITY_THRESHOLD_VERY_CLOSE" desc="The text on the Easy unlock settings page indicating the phone should be very close for the unlock to work.">
+      very close

[stevenjb, 2017/07/17 16:43:36]

Usually we would capitalize these, e.g. 'Very close'

[sacomoto, 2017/07/20 19:50:15]

Done.

+    </message>
+    <message name="IDS_SETTINGS_EASY_UNLOCK_PROXIMITY_THRESHOLD_CLOSE" desc="The text on the Easy unlock settings page indicating the phone should be close for the unlock to work.">
+      close
+    </message>
+    <message name="IDS_SETTINGS_EASY_UNLOCK_PROXIMITY_THRESHOLD_FAR" desc="The text on the Easy unlock settings page indicating the phone can be far for the unlock to work.">
+      far
+    </message>
+    <message name="IDS_SETTINGS_EASY_UNLOCK_PROXIMITY_THRESHOLD_VERY_FAR" desc="The text on the Easy unlock settings page indicating the phone can be very far for the unlock to work.">
+      very far
+    </message>
 
     <!-- Strings specific to the Settings Internet Page -->
     <message name="IDS_SETTINGS_INTERNET" desc="Name of the settings page which displays internet preferences.">