bluetooth: Fix passkey string conversion

Currently passkey is padded with trailing zeros if it's less than 6
digits. It should be padded with leading zeros instead.

Bug: 817574
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I3a27412c9c053141d28a6e7e7e609de28729ee20
Reviewed-on: https://chromium-review.googlesource.com/1005616Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Sonny Sasaka <sonnysasaka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549695}

ui/webui/resources/cr_components/chromeos/bluetooth_dialog.js

@@ -422,9 +422,9 @@ Polymer({
         (pairing == PairingEventType.DISPLAY_PASSKEY ||
          pairing == PairingEventType.KEYS_ENTERED ||
          pairing == PairingEventType.CONFIRM_PASSKEY)) {
-      var passkeyString = String(this.pairingEvent_.passkey);
-      if (index < passkeyString.length)
-        digit = passkeyString[index];
+      var passkeyString =
+          String(this.pairingEvent_.passkey).padStart(this.digits_.length, '0');
+      digit = passkeyString[index];
     }
     return digit;
   },