ZoomToFocusedInput currently does not properly handle the
case where the input element is inside a nested scrollable subframe,
as it only asks APZ to pan/zoom the root content document, and
the root content document might not be zoomable or scrollable to
make the input element visible. To handle this case properly we need
to ask the main thread to scroll the element into view first.

Differential Revision: https://phabricator.services.mozilla.com/D20983

--HG--
extra : rebase_source : 21d2b66bb657b1165820d76fe86badb7838035da
extra : histedit_source : 996b4eebf9855445c11135f5f7a3789209b720f8%2C183ffc9ae30c1df4813204dd1bedf5b9f5608518