TimelinePanel: add "Started at" value into Details pane <EOM>

BUG=326121
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/141753011

git-svn-id: svn://svn.chromium.org/blink/trunk@165438 bbb929c8-8fbe-4397-9dbb-9b2b20218538