Unpublished extension doesn't load on macOS

[srirambv]

Description

Unpublished extension doesn't load on macOS

Steps to Reproduce

1. Enable Experimental Extension API in brave://flags
2. Download and load extension from https://medium-unlimited.ml/download/
3. Doesn't allow to enable extension. Shows error message
4. Quit and relaunch, same issue

Actual result:

User recorded extension issue

Expected result:

Should enable extension

Reproduces how often:

Easy

Brave version (brave://version info)

Brave	0.62.50 Chromium: 73.0.3683.86 (Official Build) (64-bit)
Revision	f9b0bec6063ea50ce2b71f5b9abbae7beee319a6-refs/branch-heads/3683@{#858}
OS	Mac OS X

Reproducible on current release:

• Does it reproduce on brave-browser dev/beta builds?
  Yes on all channels

Website problems only:

• Does the issue resolve itself when disabling Brave Shields?
  N/A
• Is the issue reproducible on the latest version of Chrome?
  User confirmed that it works after restart on Chrome

Additional Information

Issue not reproducible on Linux. I was able to install extension without having to enable flag as well.
cc: @GeetaSarvadnya @btlechowski to verify on Windows