Avoid invoking ResourceFetcher::cachedResource() with a null URL

Invalid CSS image URLs end up with a null m_absoluteURL internally.
ResourceFetcher::cachedResource() cannot deal with null URLs so check
before calling it.

BUG=622703

Review-Url: https://codereview.chromium.org/2130673002
Cr-Commit-Position: refs/heads/master@{#404111}
2 files changed