can't login into https://riot.im due to shields blocking cross-origin requests

[jryans]

Description

A user reported that our site was broken in Brave. It appears that the Brave mode "3rd-party cookies blocked" also blocks cross-origin requests, which seems like a bug based on the setting's name.

Steps to Reproduce

1. Set cooking mode to "3rd-party cookies blocked"
2. Go to https://riot.im/app/#/register
3. You will see the error message "Unable to query for supported registration methods"

Actual result:

Expected result:

If I set to allow cookies, then I get:

Reproduces how often:

Always.

Brave version (brave://version info)

Brave | 0.57.18 Chromium: 71.0.3578.80 (Official Build) (64-bit)
Revision | 2ac50e7249fbd55e6f517a28131605c9fb9fe897-refs/branch-heads/3578@{#860}
OS | Mac OS X

Reproducible on current release:

• Does it reproduce on brave-browser dev/beta builds?
  Yes, reproduces on Dev as well.

Website problems only:

• Does the issue resolve itself when disabling Brave Shields?
  Yes, disabling the shield (or just changing the cookies part) resolves it.
• Is the issue reproducible on the latest version of Chrome?
  No, Chrome works correctly.

[jeroenev]

have been noticing the same for about a day
other example is https://deltablue.cloud/#/account/login

[iefremov]

Should fixed as a duplicate of #2252

[btlechowski]

Verification passed on

Brave	0.58.20 Chromium: 71.0.3578.98 (Official Build) (64-bit)
Revision	15234034d19b85dcd9a03b164ae89d04145d8368-refs/branch-heads/3578@{#897}
OS	Windows 7

Verification PASSED on Mint 19.1 x64 VM using the following build:

Brave	0.58.20 Chromium: 71.0.3578.98 (Official Build) (64-bit)
Revision	15234034d19b85dcd9a03b164ae89d04145d8368-refs/branch-heads/3578@{#897}
OS	Linux

• registered for a new account without any issues
• logged into my personal account and ensured that I could login without any issues

Verified passed with

Brave	0.58.20 Chromium: 71.0.3578.98 (Official Build) (64-bit)
Revision	15234034d19b85dcd9a03b164ae89d04145d8368-refs/branch-heads/3578@{#897}
OS	Mac OS X