Adding pref to store the user-selected proximity threshold.

chrome/app/settings_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Settings-specific strings (included from generated_resources.grd). -->
 <grit-part>
   <!-- Main Page -->
   <if expr="chromeos">
     <message name="IDS_SETTINGS_SECONDARY_USER_BANNER" desc="Banner displayed in settings page when the user is secondary in a multi-profile session.">
       Some settings belonging to <ph name="PRIMARY_EMAIL">$1<ex>john@google.com</ex></ph> are being shared with you. These settings only affect your account when using multiple sign-in.
     </message>
   </if>
 
@@ skipping 1088 matching lines @@
     </message>
     <message name="IDS_SETTINGS_EASY_UNLOCK_TURN_OFF_ERROR_TITLE" desc="The title text on the Easy unlock turn off dialog when Easy unlock could not be turned off because there is a server error.">
       Smart Lock is unavailable
     </message>
     <message name="IDS_SETTINGS_EASY_UNLOCK_TURN_OFF_ERROR_MESSAGE" desc="The text on the Easy unlock turn off dialog when Easy unlock could not be turned off because there is a server error.">
       Smart Lock is currently unavailable. Please try again later.
     </message>
     <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
+    </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

[battre, 2017/07/12 07:18:47]

Drive by: I think that these values are pretty meaningless to me as a user.

[sacomoto, 2017/07/12 15:52:52]

This is intentionally vague. Unfortunately, the mapping between RSSI value and
distance is not completely accurate, e.g. we cannot say that -70 RSSI
corresponds to 1m.

The signal strength is intrinsically noisy and the geometry (e.g. the location
of the BLE antenas) affects the signal strength. Moreover, the value of the RSSI
has a big dependency on the hardware combination (phone and the Chromebook).

+    </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.">
       Network
     </message>
     <message name="IDS_SETTINGS_INTERNET_CONFIG" desc="Name of the settings page for configuring a network.">
       Configure network
     </message>
     <message name="IDS_SETTINGS_INTERNET_DETAIL" desc="Name of the settings page which displays network details.">
       Network Details
@@ skipping 2074 matching lines @@
       System
     </message>
     <message name="IDS_SETTINGS_SYSTEM_HARDWARE_ACCELERATION_LABEL" desc="Label for the checkbox that forces Chrome to render via hardware acceleration (GPU) when available.">
       Use hardware acceleration when available
     </message>
     <message name="IDS_SETTINGS_SYSTEM_PROXY_SETTINGS_LABEL" desc="Label for the control that opens a user's network proxy settings.">
       Open proxy settings
     </message>
   </if>
 </grit-part>