GitLab

This is reland of https://crrev.com/c/1164570

Change summary: A header view may outlive the target widget,
which can be differet widget. Reset the reference and check
before calling target widget.

Bug: b/112240082
Test: covered by unittests
Change-Id: Ia98d44a487b6cd2077b110e4a9799780cdbc4ebe
Reviewed-on: https://chromium-review.googlesource.com/1165697
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581350}