Verified locally that reading the manifest does not affect performance in Fenix
(which uses 5 different built-in extensions) for google.com.

79.0.20200617184425 - Don't parse manifest

fully-drawn : avg: 2030 ± 11 stddev: 112 n=101
about:blank : avg: 2775 ± 26 stddev: 265 n=101
page loaded : avg: 3922 ± 50 stddev: 503 n=101

79.0.20200617205116 - Parse manifest

fully-drawn : avg: 2025 ± 13 stddev: 130 n=89
about:blank : avg: 2761 ± 24 stddev: 232 n=89
page loaded : avg: 3866 ± 39 stddev: 377 n=89

Differential Revision: https://phabricator.services.mozilla.com/D80567