Avoid RefPtr churn

This is a speculative fix for 560890, but seems like it might be a
good idea anyway. We have a reference to the thing having a
reference. No need to add another reference on top of that.

In the process drop a null check in StyleFetchedImageSet::image(),
ImageResource::image always returns an Image object. If nothing else
the Image::nullImage().

BUG=560890

Review URL: https://codereview.chromium.org/1472253003

Cr-Commit-Position: refs/heads/master@{#361640}
3 files changed