Chrome: 64.0.3273.3 (Official Build) canary (64-bit) (cohort: Clang-64)
Firefox: 52.4.1 (64-bit)

Steps to repro: Tested in Chrome and Firefox
# Visit https://www.google.com/chrome/browser/desktop/index.html?brand=CHBD
# Click download
# Press tab to cycle between elements in the modal
# Notice how navigation wraps around and focus never leaves the modal
# Press shift+tab to cycle between elements in the modal
# Notice how navigation wraps around and focus never leaves the modal
# Press alt+d to jump to the omnibox
# Press tab multiple times and observe where the focus is
# Notice that we have escaped the modal and are tabbing behind the modal

Expected: When focus escapes the modal, it should be brought back to the first focusable element in the modal.

Actual: Focus is allowed to remain behind the modal.