[DL] Activate locked elements when focusing happens

When a locked element needs to be focused, for example with the focus()
method or through tabindex navigation, we will send the beforeactivate
event and focus the element.

See doc: http://bit.ly/2RXULVi

Bug: 882663
Change-Id: I1320c41f5d08e148f52a97480d2cc9833c43fb08
Reviewed-on: https://chromium-review.googlesource.com/c/1477146
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633441}
4 files changed