Fix null pointer exception when attempting to load an image without the target

- properly annotated TracingModel.Event fields to let compiler catch the problem;
- check target is not null before using it.

BUG=424503

Review URL: https://codereview.chromium.org/658363002

git-svn-id: svn://svn.chromium.org/blink/trunk@183889 bbb929c8-8fbe-4397-9dbb-9b2b20218538