Remove obsolete method in WebDOMActivityLogger

This patch cleans up the API in WebDOMActivityLogger; it is the final
patch in the series consisting of:
  https://codereview.chromium.org/23817008/ (in Blink)
  https://codereview.chromium.org/24243011/ (in Chromium)
and is intended to land only after those two other patches.

BUG=294500

Review URL: https://chromiumcodereview.appspot.com/24247009

git-svn-id: svn://svn.chromium.org/blink/trunk@158338 bbb929c8-8fbe-4397-9dbb-9b2b20218538