Avoid exception when building details for DecodeImage event

Url for this event is retrieved from imageURL field later in the same switch.

BUG=361045

Review URL: https://codereview.chromium.org/339603002

git-svn-id: svn://svn.chromium.org/blink/trunk@176227 bbb929c8-8fbe-4397-9dbb-9b2b20218538