GitLab

Moves the transient parent utilities to gtk2_util and makes both
dialogs use it.

This is a reland of r259318 with a fix of test failures.

BUG=334331
TBR=erg@chromium.org
TEST=manually

Review URL: https://codereview.chromium.org/221813010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@261473 0039d316-1c4b-4281-b951-d872f2087c98