Refactor ScopedFocusNavigation

- Make the function names shorter
- Make ScopedFocusNavigation::createFor() takes only one parameter, instead of two, because it receives the same elements

BUG=531990

Review URL: https://codereview.chromium.org/1805483002

Cr-Commit-Position: refs/heads/master@{#381209}
1 file changed