Change InternalSettings to reset overriden default fonts

Change the InternalSettings::Backup::restoreTo to reset the font
family mappings as a part of the restore option.

This fixes the flakiness that occurs when a pixel test with a matching
script is executed after a test that overrides the default font
family for that script.

BUG=305018
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/26122004

git-svn-id: svn://svn.chromium.org/blink/trunk@159167 bbb929c8-8fbe-4397-9dbb-9b2b20218538