Add rounded corners to macOS/Linux custom ad notifications

[tmancey]

No description provided.

[tmancey]

Resolved as part of #13132, please test as part of the new custom ad notifications

[stephendonner]

Verified PASSED on macOS using

Brave	1.26.7 Chromium: 90.0.4430.93 (Official Build) nightly (x86_64)
Revision	4df112c29cfe9a2c69b14195c0275faed4e997a7-refs/branch-heads/4430@{#1348}
OS	macOS Version 11.3.1 (Build 20E241)

and compared to:

Brave	1.25.43 Chromium: 90.0.4430.85 (Official Build) nightly (x86_64)
Revision	5bc145d831c180d9ff94f29a0d7a2e1cbd30ef36-refs/branch-heads/4430@{#1311}
OS	macOS Version 11.3.1 (Build 20E241)

Steps:

1. new profile
2. launched with --enable-logging=stderr --vmodule="*/bat-native-ledger/*"=6,"*/brave_rewards/*"=6,"*/bat-native-ads/*"=6,"*/bat-native-confirmations/*"=6,"*/brave_ads/*"=9,"*/brave_user_model/*"=6 --brave-ads-staging --brave-ads-debug --rewards=staging=true,reconcile-interval=5
3. clicked through the Welcome page
4. enabled Rewards
5. opened a few new-tab pages
6. waited
7. confirmed I got the system notification for ads

rounded, ads staging (1.26.7)	rounded, ads prod (1.26.7)	square, ads staging (1.25.43)	square, ads prod (1.25.43)

---

Verification passed on

Brave	1.26.45 Chromium: 91.0.4472.77 (Official Build) beta (64-bit)
Revision	1cecd5c8a856bc2a5adda436e7b84d8d21b339b6-refs/branch-heads/4472@{#1246}
OS	Ubuntu 18.04 LTS

Verified that ads have rounded corners on Linux