Bug 1306634 Part 1 - Handle a long press to select a word in an unfocused iframe. r=mats,marionette-reviewers,whimboo

Long-pressing on a text in an unfocused iframe to select a word never
works. Currently, you need to single tap to focus the iframe first.

Each PresShell has an associated AccessibleCaretEventHub. This patch
fixes this bug by routing the event to the AccessibleCaretEventHub under
the event point, and handle it there. If the event is not handled, then
we handle it by the focused AccessibleCaretEventHub as before.

I've experimented with only routing the event to the
AccessibleCaretEventHub under the event point, without routing to the
fallback focused AccessibleCaretEventHub. However, caret dragging didn't
work in iframes. I didn't debug further.

Differential Revision: https://phabricator.services.mozilla.com/D52767

--HG--
extra : moz-landing-system : lando