Bug 1642629 - Reserve items in SegmentBuilder before simple loops. r=kvark
Bug 1642629 - Reserve items in SegmentBuilder before simple loops. r=kvark

Differential Revision: https://phabricator.services.mozilla.com/D80192

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/7cbc9b5f00f1b83e25c19be1d76e6a404415180d

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender