Remove StyleRuleBase::m_sourceLine

This field had been used only by the DevTools CSS selector profiler,
which no longer exists.

R=eseidel, vsevik, yurys@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21133005

git-svn-id: svn://svn.chromium.org/blink/trunk@155233 bbb929c8-8fbe-4397-9dbb-9b2b20218538