[Autofill] Merged FormData::child_frames, child_frame_predecessors.

This CL merges the two members
- FormData::child_frames
- FormData::child_frame_predecessors
into one vector.

This simplifies simultaneous manipulation of the vectors.
It also eradicates different vector sizes as error source.

Otherwise, this CL preserves equivalence.

Bug: 1187842, 1187857
Change-Id: I7940d7f7131ad81785ab26b33df60496886c742d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2978633
Reviewed-by: Matthias Körber <koerber@google.com>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/master@{#896664}