Console errors when opening Private Window

[btlechowski]

Follow up to #6872

Steps to Reproduce

1. Change language to en_GB
2. Open a Private Window

Actual result:

[3184:3184:1112/181859.940022:ERROR:message_formatter.cc(91)] MessageFormat({0, plural,
      =1 {Private}
      other {{Private (#)}
    }) failed with U_PATTERN_SYNTAX_ERROR
[3184:3184:1112/181859.940473:ERROR:message_formatter.cc(91)] MessageFormat({0, plural,
      =1 {Private}
      other {{Private (#)}
    }) failed with U_PATTERN_SYNTAX_ERROR
[3184:3184:1112/181859.940575:ERROR:message_formatter.cc(91)] MessageFormat({0, plural,
      =1 {Private}
      other {{Private (#)}
    }) failed with U_PATTERN_SYNTAX_ERROR
[3184:3184:1112/181859.955558:ERROR:message_formatter.cc(91)] MessageFormat({0, plural,
      =1 {Private}
      other {{Private (#)}
    }) failed with U_PATTERN_SYNTAX_ERROR

Also strings are lower case, which is contrary to #6872

Expected result:

No error

Reproduces how often:

Always

Brave version (brave://version info)

Brave	0.71.115 Chromium: 78.0.3904.97 (Official Build) (64-bit)
Revision	021b9028c246d820be17a10e5b393ee90f41375e-refs/branch-heads/3904@{#859}
OS	Ubuntu 18.04 LTS

cc @brave/legacy_qa @petemill @mkarolin @rebron

[btlechowski]

Talked to @petemill and according to him this is fixed by brave/brave-core#3982. Setting as a dupe of #6890