Add support to the feedback app for chrome://slow

BUG=285522
R=mpcomplete@chromium.org, rkc@chromium.org, xiyuan@chromium.org

Review URL: https://codereview.chromium.org/23677015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224841 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224841