Clean up copy&paste in confirmation dialogs for prefs

This CL consolidates the three implementations of virtually identical
confirmation dialogs for prefs into a single class. The new code has
the further advantage that the confirmation dialogs no longer know about
and manipulate any input elements directly but instead, use the dialog
pref system to commit or roll back changes when they are confirmed or
cancelled.

BUG=157319


Review URL: https://chromiumcodereview.appspot.com/11232059

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@163874 0039d316-1c4b-4281-b951-d872f2087c98
9 files changed