GitLab

This is done by refactoring some tests to observe focus changes in a
cross-platform manner, and by converting some custom-crafted events to
use the cross-platform event generator.

For now, the code that is only enabled on Windows is unchanged. This
still has aura dependencies, but I hope to enable some of these tests on
Mac in future.

BUG=399191

Review URL: https://codereview.chromium.org/444583002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287944 0039d316-1c4b-4281-b951-d872f2087c98