[omnibox chrome:omnibox] Add accesskeys for import and export buttons

The 3 buttons below were the only remaining inputs that did not have accesskeys.
- Copy as te(x)t
- Do(w)nload as JSON
- I(m)port JSON

Bug: 891303
Change-Id: I10930f0e9b00a1625b14547a211007adb2aef095
Reviewed-on: https://chromium-review.googlesource.com/c/1409912
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623349}
1 file changed