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

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=183889

[caseq, 2014-10-17 11:58:10]

caseq@chromium.org changed reviewers:
+ pfeldman@chromium.org, yurys@chromium.org

[caseq, 2014-10-17 11:59:30]

Note the check for null is later than it could be to keep the closure compiler
happy (I don't think we care to avoid extra work associated with fetching
contents of a snapshot since this is more like an exception).

[yurys, 2014-10-17 12:30:43]

lgtm

[caseq, 2014-10-17 12:34:19]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2014-10-17 12:34:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/658363002/1

[commit-bot: I haz the power, 2014-10-17 13:37:27]

Committed patchset #1 (id:1) as 183889