Set the experiment/variation info in crash reports using the crash key logging system.

BUG=77656
R=asvitkine@chromium.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/23995025

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225186 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225186