Profile Reset: support "soft reset" of search engines, homepage and startup pages. The soft reset doesn't clear user-defined search engines, homepage URL and startup URLs.

BUG=235037

Review URL: https://codereview.chromium.org/23496004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226880 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226880