Crashes occur because the callback is not set and was attempting to fire the null callback. Fix is to move assignment of the callback earlier to prevent calls to a unassigned callback.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3d71e5ccae71a73e30f7901d5eab7ce38f64f53d

commit 3d71e5ccae71a73e30f7901d5eab7ce38f64f53d
Author: Jimmy Gong <jimmyxgong@google.com>
Date: Thu Nov 01 23:42:58 2018

Fix crash caused by null callback in MdnsHostLocator

- Fixes issue where switching wifi networks while starting Mdns
  HostLocator would crash chrome.

Bug:  chromium:900779 
Test: end to end
Change-Id: Ide5e1bb53961dcd654aedadbffc6cf2a83c5d998
Reviewed-on: https://chromium-review.googlesource.com/c/1313331
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Commit-Queue: jimmy gong <jimmyxgong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604753}
[modify] https://crrev.com/3d71e5ccae71a73e30f7901d5eab7ce38f64f53d/chrome/browser/chromeos/smb_client/discovery/mdns_host_locator.cc

This is a fix for a chrome crash, would like like to merge to m71.

Per the M71 Branch Announcement: "Have your change checked into the trunk, and tested by you on either a Canary build or Dev channel release before asking for merge approval."   Has this testing taken place?  If so, results?  If not, please test prior to the merge request.  Thanks.

Sorry - we just wanted to try and make sure it would get to 71. Chrome uprev'd last night without this change included so we aren't sure when the next uprev will be.

It has been tested by us locally and is a low risk change. We'll wait a fewe more days and see if Chrome uprevs by then.

This bug requires manual review: M71 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: benmason@(Android), kariahda@(iOS), kbleicher@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Merge approved for ChromeOS M71

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c14ec990a2bcb3593713042603983f2d55156756

commit c14ec990a2bcb3593713042603983f2d55156756
Author: Jimmy Gong <jimmyxgong@google.com>
Date: Mon Nov 05 23:34:00 2018

Fix crash caused by null callback in MdnsHostLocator

- Fixes issue where switching wifi networks while starting Mdns
  HostLocator would crash chrome.

Bug:  chromium:900779 
Test: end to end
Change-Id: Ide5e1bb53961dcd654aedadbffc6cf2a83c5d998
Reviewed-on: https://chromium-review.googlesource.com/c/1313331
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Commit-Queue: jimmy gong <jimmyxgong@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#604753}(cherry picked from commit 3d71e5ccae71a73e30f7901d5eab7ce38f64f53d)
Reviewed-on: https://chromium-review.googlesource.com/c/1318743
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/branch-heads/3578@{#529}
Cr-Branched-From: 4226ddf99103e493d7afb23a4c7902ee496108b6-refs/heads/master@{#599034}
[modify] https://crrev.com/c14ec990a2bcb3593713042603983f2d55156756/chrome/browser/chromeos/smb_client/discovery/mdns_host_locator.cc

The following revision refers to this bug: 
https://chromium.googlesource.com/chromium/src.git/+/c14ec990a2bcb3593713042603983f2d55156756

Commit: c14ec990a2bcb3593713042603983f2d55156756
Author: jimmyxgong@google.com
Commiter: zentaro@chromium.org
Date: 2018-11-05 23:34:00 +0000 UTC

Fix crash caused by null callback in MdnsHostLocator

- Fixes issue where switching wifi networks while starting Mdns
  HostLocator would crash chrome.

Bug:  chromium:900779 
Test: end to end
Change-Id: Ide5e1bb53961dcd654aedadbffc6cf2a83c5d998
Reviewed-on: https://chromium-review.googlesource.com/c/1313331
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Commit-Queue: jimmy gong <jimmyxgong@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#604753}(cherry picked from commit 3d71e5ccae71a73e30f7901d5eab7ce38f64f53d)
Reviewed-on: https://chromium-review.googlesource.com/c/1318743
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/branch-heads/3578@{#529}
Cr-Branched-From: 4226ddf99103e493d7afb23a4c7902ee496108b6-refs/heads/master@{#599034}