Trace base class Event in SpeechRecognitionError.

R=sigbjornf@opera.com
BUG=

Review URL: https://codereview.chromium.org/183683007

git-svn-id: svn://svn.chromium.org/blink/trunk@168133 bbb929c8-8fbe-4397-9dbb-9b2b20218538