Enable Activity Log to log creation of event handlers by extension content
scripts, and disable logging of event handler assignments.

BUG=388287

Review URL: https://codereview.chromium.org/333713006

git-svn-id: svn://svn.chromium.org/blink/trunk@176883 bbb929c8-8fbe-4397-9dbb-9b2b20218538