Follows-up
5b0749d, which first introduced a call to performance.mark
outside of startup.js – thus triggering this excotic bug in Safari.
In Safari the native thread MAY (emphasis on MAY) decide at any
point to recreate window.performance and blow away any previously
succesfully attached properties.
Bug: T112287
Change-Id: I653e580566da5335d34f1cf1ef52f51ceee2cf5a