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
+    </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.">