I'm not sure why the usual comment saying that a CL has landed for this bug wasn't added, but I landed a fix [1] yesterday and verified it in Mac Canary today. So I'd like to request the fix to be merged into M58.

[1] https://codereview.chromium.org/2736863002/

Your change meets the bar and is auto-approved for M58. Please go ahead and merge the CL to branch 3029 manually. Please contact milestone owner if you have questions.
Owners: amineer@(clank), cmasso@(bling), bhthompson@(cros), govind@(desktop)

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

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a0d87961d1bf1d99aa7a82d5166be3aef1ec75f4

commit a0d87961d1bf1d99aa7a82d5166be3aef1ec75f4
Author: Derek Cheng <imcheng@chromium.org>
Date: Fri Mar 10 18:35:53 2017

Migrate the pref to always show the Cast icon

This CL fixes the bug in which the user preference to pin the Cast toolbar icon
disappears due to the lack of migration from the old pref to the new pref.

BUG= 698931 

Review-Url: https://codereview.chromium.org/2736863002
Cr-Commit-Position: refs/heads/master@{#455525}
(cherry picked from commit fb4e8ac40deaad4025dcacd96c9be4fbf2b7d193)

Review-Url: https://codereview.chromium.org/2744883002 .
Cr-Commit-Position: refs/branch-heads/3029@{#115}
Cr-Branched-From: 939b32ee5ba05c396eef3fd992822fcca9a2e262-refs/heads/master@{#454471}

[modify] https://crrev.com/a0d87961d1bf1d99aa7a82d5166be3aef1ec75f4/chrome/browser/prefs/browser_prefs.cc
[modify] https://crrev.com/a0d87961d1bf1d99aa7a82d5166be3aef1ec75f4/chrome/browser/ui/views/media_router/media_router_ui_browsertest.cc

verified in 59.0.3047.4