[DL]: Add activatable flag to DisplayLockOptions

Adds the boolean "activatable" flag to DisplayLockOptions, which is
false by default. Signifies whether the lock can be activated by the
browser when needed, in cases like find-in-page, tab order navigation,
URL fragment anchor, etc.

This CL also renames the "IsSearchable" method in DisplayLockContext
to "IsActivatable", to be consistent with the option name.

The actual activation & "beforeactivate" event will be implemented in
another CL.

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

Bug: 882663
Change-Id: I259ff7f2fed2e3e1f446fa33f202ed2d81a76ad7
Reviewed-on: https://chromium-review.googlesource.com/c/1466544
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632383}
7 files changed